Offer rate vs Acceptance rate

What is the different between offer rate and acceptance rate?

Also, if the offer rate is to high but the acceptance rate not to high e.g 1000 who got offer and 120 acceptance

Is that mean the offer rate not that important?

The offer rate is the number of offers a university makes divided by the number of applications the receive (for a given course). However, universities make more offers that they have places, because they know that you're also applying elsewhere (typically up to four other places), so they have to over-offer. (Some universities over-offer more than others; I suspect that neither Oxford nor Cambridge over-offer at all, as almost everyone who is offered a place there will make it their firm choice.)
Also, even if you do make a particular university your firm or insurance choice, you may end up missing the grade requirements and securing a place via Clearing instead, or changing your mind about going to university entirely. So the acceptance rate (the number of people accepted onto a course divided by the number of applications) is actually measuring something completely different - and isn't terribly useful. It's more a measure of how "in demand" a course is.
If you're trying to work out the likelihood that you'll receive an offer (which is what most people are interested in) then you're interested in the offer rate.
If the offer rate is high, but the acceptance rate is low, it means that they make way more offers than that have places because they know students tend to choose to go elsewhere. Either that or they make offers that students are unlikely to hit, so a lot of students end up missing the offer grades.

I am interesting in a course that got 704 offer rate and the acceptance rate is 81.

Is that mean I have a good chance or bad?

A rate is normally expressed as a percentage or as a ratio rather than as a simple number. If it rains for 24 out of the 30 days in April then the rate of rainy days is 80% (24/30) or 4:5 (4 rainy days for every 5 days).
To say that a course has a "704 offer rate" makes no sense. Perhaps you mean that they made 704 offers, in which case to work out the offer rate, you need to know how applicants there were. (If they made 704 offers to 750 applicants then the offer rate is high, and any individual has a good chance of an offer. If they made 704 offers to 7,500 applicants then the offer rate is low, and any individual has poor chance of an offer.)
Do you have a link to the source of those numbers? (Looking at the source might make it clearer what they mean.)

The applications number is 993 and the number of offers is 703 and the number of acceptances is 83.

Is that meaning I have a bad chance ?

No. It means that the average applicant has a 703 / 993, or 71%, chance of receiving an offer. That's what you want right now - an offer.
Those are pretty good odds. (Obviously, your individual grades, personal statement, reference, etc. will move your individual odds up or down.)
It also means that the vast majority of those who receive an offer opt not to go there (presumably as they also have other, "better", offers).

But if I already have the grades (1st year student at another university, so I already have my foundation grades) I should get the acceptance immediately right?

What do you mean by your "foundation grades"? Do you mean grades from a foundation year at another uni? That isn't a qualification, and is of zero value to most unis (other than the one you're at).
If your A level grades (or BTECs, or whatever) meet their entry requirements then you have a high likelihood of an offer - but it's not guaranteed.

I meant I am an international student and already studied foundation year from one of the institute and now I am taking my first year.

However, I decided to change my course and uni so I applied to another Uni starting from1st year again.

That's what I am asking in this case. Should I take the acceptance immediately instead of the offer?

I have absolutely no clue what "Should I take the acceptance immediately instead of the offer?" means.
If you meet the entry requirements for the new course / uni then you are likely to receive an offer.
Given that you already have qualifications (International A level, IB, whatever) then that offer is likely be unconditional.

Sorry for the confusion. The requirements AAA - ABB and my grades AAB. I applied for 1st year because it is another course that not related to the one that I am in.

I don't know my chances to be accepted, but I just asked that normally students get offer with requirements like the grades. However, I already have the grades from the previous year, so I am not sure what will happen to me.

When a uni gives a range of grades (e.g. "AAA - ABB") it normally means that the first set is the "standard" offer and the second set is the "contextual" offer.
Assuming that this is how this particular uni works, and assuming that you are not a contextual candidate, then your AAB grades don't meet their AAA requirement. That being the case, all you can do is wait and see whether or not you receive an offer.

When a uni gives a range of grades (e.g. "AAA - ABB") it normally means that the first set is the "standard" offer and the second set is the "contextual" offer.
Assuming that this is how this particular uni works, and assuming that you are not a contextual candidate, then your AAB grades don't meet their AAA requirement. That being the case, all you can do is wait and see whether or not you receive an offer.

But answer me honestly in this case, is there a chance to be accepted?