can someone explain scope collimation to me?

I have a friend who sent their scope back to nikon because he was having issues with it staying dialed in on his 35 whelen. They returned it and the sheet said something about they adjusted the scope's collimation, or something like that. They didn't really go into more detail.

I googled it and all I could find was a defintion saying it was to make parallel, or line up. Something along that line.

It still doesn't really tell me how that would affect how a scope stays sighted in.

Maybe it has to do with the mirrors inside the scope? I dunno. Anyone know? Thanks!