What is implantation bleeding??

(If I spelled that correct,sorry if not) but I had sex recently with withdrawal and my period is supposed to be in 6-7 days, Goow says I have a 3% chance of getting pregnant and I had ovulation a week ago. But someone said implantation bleeding. My periods are very very heavy so if I have one I feel like it's safe to say that I'm not but I still don't know what implantation  bleeding is.