What is implantation bleeding exactly?

My cycles aren’t very regular or consistent. I started bleeding super light for barley 2 days and then various spotting of dark brown discharge. I have been various pregnancy symptoms but I realized those same symptoms are also common with pre-period/PMS stuff. My boobs specially I have been slightly more sensitive than usual. I have taken multiple pregnancy test before the bleeding and they were all negative.

I don’t remember before when I was pregnant if I had any bleeding or not. I also just got my IUD removed in August.