I don't understand.. HELP!!

I don't undertand the most fertile days in a cycle. So I just read online that in order to further the possible chance of pregnancy one must have intercourse at least two to four days before ovulation. now my biggest question is... here on the <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> the green for ovulation, should one per say be having sex 2 days before it's green, or the very first day of it being green?