Someone help me out with this app?

Starr
With the overview of my period, it has like a 'fertile window'? What is that exactly? I mean, I can assume it's to do with my vagina and shit but should I avoid having sex then? Do I have sex? I reeeeeaaaaallly don't want kids right now and I'm taking all of the precautions I can.
Edit: I'm not on anymore form of birth control right now due to horrible experience with past methods, but I use condoms, which I know isn't really reliable but I don't really get a choice lol