What exactly is 'revision'?

I am looking into all of the different surgeries to see what is right for me and I look to see what revision is and it says there is like no info on it...if you know anything about it please let me know...tia!!

Revision is when you have had some type of bypass surgery before and for some reason it does not work anymore or you have complications from the first surgery that the surgeon has to go in and fix. Julie