Period/sex

Ok I don’t want to sound stupid or slow but never had a sex Ed class and my mom didn’t explain these things to me ever ... but someone please explain this to me , whats the difference between being fertile and ovulating.... and which one can you be on and have sex and chances of you being pregnant are very low and which can you be on the chances of you being pregnant are high... if that makes since and can you have sex the 1-3 days after your period has ended. And if you take plan b the next night after you had unprotected sex ... and he only cam a little inside you .... would u be okay since it wasn’t a lot and you took emergency conception...