Might be a stupid question

What is implantation bleeding? I heard a doctor say it to me before but I never got the chance to actually ask him.....my periods are crazy I once didn't have a period for 6 months and I wasn't pregnant....I also have had periods where I had a period for a week then stopped for 4 days then started  another one for a week quit for 3 days started another one for a week they checked for pregnancy and miscarriage and I wasn't pregnant nor miscarriage so they said I have dysfunctional uterine bleeding.....which just means a really bad period has anyone else been through this too??