Someone please explain what is an MSci?

I’ve heard of MSci degrees for natural sciences, chemistry and biology before and I am confused about what they really are. Do you apply to them at the same time as applying to uni and are the entrance requirements any higher or are they applied to after first year?
Also if I am applying for an MSCi which has a study abroad component do I need to have the A Level in a language?

The universities and courses I am looking into are the following:
. Imperial
. Cambridge
. Kings college London
. Bristol

I’ve heard of MSci degrees for natural sciences, chemistry and biology before and I am confused about what they really are. Do you apply to them at the same time as applying to uni and are the entrance requirements any higher or are they applied to after first year?
Also if I am applying for an MSCi which has a study abroad component do I need to have the A Level in a language?

The universities and courses I am looking into are the following:
. Imperial
. Cambridge
. Kings college London
. Bristol

An MSci is an "Integrated Master's" degree - it's a undergraduate (bachelor's) degree with a Master's degree bolted on the end.
At some universities the entry requirements for the MSci will be higher than for the BSc. At some they will not. So check the web sites of the unis you've mentioned to find out.
At some unis, you can opt into (or out of) the extra MSci year (i.e. over and above the three BSc years). Some also have academic targets you need to hit to be allowed into the MSci year. This will vary by university, so check what it says in their web site.