What is sleepimage?

I was looking in diskinventoryx and found a plain text file called sleepimage. It is 4GB!! can someone please tell me what this is so I can delete it if I don't need it. THX

It is the image that your system stores to RAM when you put your computer to sleep, so that when you wake it up all of your previous work is still open. It is the same size as the amount of installed RAM in your computer. Without it you would lose everything you were doing when you put your computer to sleep, and would have to reboot every time. If you delete it it will just show up again the next time your computer goes to sleep, so you might as well leave it alone.