Filed my federal tax return and and did not know of a 1099-r. What is a 1099-r and how does it work? What can I do if I already filed my federal tax return.

I filed my Federal and one state tax forms (I have more than one state tax form). I had thought I had received all of my forms that were necessary for filing, but then I had discovered that I had a 1099-r form. I did a bit of research on the form as I've never seen it before and it looks like I would get this form if there was a rollover and or if I withdrew money from a retirement account. Now I did not withdraw but I wanted to ask what does rollover mean? Did my previous employer send over my 401k (which this is what the form must be for because I don't have any other retirement accounts) to my new employer? Or is this something I have to fill out every year if I have payment elections going to a 401k? If so, that means I would have two 1099-r forms (I had two jobs total in 2023). So I already filed but left off my 1099-r form(s). My status is pending currently as I filed 3 days ago. What can I do? The retirement money amount was small (did not exceed 500 dollars) and I did not touch the money. Would it matter if it remained left off of my taxes?