[X4U] What is private>var>vm>swapfile?

[Mark Des Cotes]

I'm using WhatSize to help me clean out my hard drive. I notice an  
invisible folder on the HD called "private" that is 2.3 GB in size.  
The main culprit inside is a folder called "var" which in turn  
contains the folder "vm" which holds a bunch of files titled  
"swapfile#" Can someone tell me what these folders are for? Do they  
need to be that big?

thanks

Mark