Okay, so, the implant is among various hormonal methods of contraception (others being the pill, the injection, the patch, and the coil (though there is a non-hormonal coil too)). These are called hormonal because they release hormones into your system which prevent you from ovulating (releasing an egg). As no eggs are released, it is not possible for you to become pregnant even when a guy ejaculates inside you, because there is nothing for the sperm to fertilise.
HOWEVER
No hormonal method of contraception will guarantee that you will not fall pregnant, though they are more than 99% effective, so realistically there is no need to use condoms in conjuction with them unless the condom is to protect against STDs or you would like that extra level of protection against pregnancy.
If the implant would be the first form of hormonal contraception you've been on, I would recommend that you do a lot of research on it, as it can affect women very differently, particularly in how they experience their periods. You may stop having periods while you have the injection, which isn't very reassuring when you want to see evidence that you're not pregnant!
I started - and have stayed - on the pill. I like that I bleed between packs of it so that I know I'm not pregnant, and that I can run packs together if it would be inconvenient to have a period at the usual time (e.g. if I'm going on a beach holiday). As long as you can trust yourself to remember to take it every day, you won't have any problems. And if you suffer any adverse side effects (research on possible side effects should be part of your decision as to which contraceptive will be best for you) it's easier to switch to a different pill than it is to a different implant.
Just get informed, then chat to your doctor.