What is this file?
You can open it in property list editor and have a look — It stores things like your workspace/project document layouts, nothing you would lose sleep over if lost.
Can I ignore this file?
You would ignore it in all but exceptional cases. It’s easily reconstructed, and should be considered local to your system’s user account. To stretch imagination or reason to track it: If you worked by yourself and mirrored your user accounts, then you might want this information synchronized among your machines. Sure, you could track it, but it would produce a lot of unnecessary revision control noise.