What is kernel_task? And why does it use so much memory?

I have an iMac 27 inch late 2009 purchased new in July 2010. It has a 3.08 Ghz Intel Core 2 Duo Processor and 4 GB of Memory. I recently upgraded to Yosemite and just now was unable to save an Excel document to an external hard drive with plenty of space with the warning I did not have enough memory. I went to my activity monitor and indeed there is very little memory and at the top of the usages of memory is kernel_task? I don't know what that is and why it requires so much system memory.

Also, there is an installation I aborted for an app I realized I didnt need and for some reason something it out there still taking system memory and I get a message when I shut down that it can't install. How do I delete that.