ram is memory, what it does is stores data from your drive to your CPU.
so when you want it latter its quicker, it basically tells you how much data is stored.
depends on the applications your running - for instance the side bar you use uses approximately 22k memory to run -(does on mine) if you turn it off, ur pc no longer needs to use the ram and will drop the percentage slightly - hit crtl-alt-delete and start your task manager - click the processes tab and you can see all the apps that require you random access memory