RAM (or memory) is used by applications and programs to store/access data in the short-term so that it can be accessed quickly. Note that this is completely different from storage space where you typically keep your files, games, etc.

The more things you have running at the same time, the more RAM will be used up. Since most machines within the last few years or so come with at least 4 GB of RAM (the minimum spec listed on ProctorU's requirements), the first thing I'd try is to close all other non-essential programs to get your RAM utilization under 90%.

Hope this helps!

Edit

You can see your RAM utilization in Windows by opening Task Manager (Ctrl + Shift + Esc) and clicking on the "Performance" tab at the top. You should see your memory usage along the left hand side. The process should be similar for a Mac.