What are Distribution updates?

[Jim Byrnes]

Running Ubuntu 12.04

Update manager just popped up with Distribution updates checked and 
under it was a file locking library for Python.

It most likely has happened before but I don't recall seeing it. 
Googling on Ubuntu Distribution updates pulls up a bunch of stuff about 
distribution upgrades, which I don't want to do.

So what is a Distribution update?

Thanks,  Jim