What is .xsession-errors?

Project:Linux software
Component:Documentation
Category:feature request
Priority:normal
Assigned:Unassigned
Status:active
Related pages:#5416: X.org Server
Tags:X.org server
Description

What is .xsession-errors?
What is the content for?
Should users ever need to be concerned about it?

Context for me asking:
I just experienced an abnormal growth of that file, which filled up my filesystem:
#5417: knotify4 fills up filesystem with .xsession-errors