Physical Ram

It's actual RAM (random access memory) you have installed. Go to the Apple menu and select "About This Mac", it will say under "Memory" how much you have.

 

Physical RAM refers to the memory chips inside your computer, as opposed to virtual memory which is written temporarily to your hard drive. You can see the amount of physical RAM you have under  -> About This Mac -> Memory.

 

All that pretty typing, and BlueRevolution beat me to it.

You can also click on the "More info..." button to open System Profiler. In the left column, under Harware, click on Memory to see more specific information about your RAM.

I hope that helps!

 

You also need a fast enough CPU - I believe it's 867 mhz or something like that, not just RAM. Otherwise Leopard will not install.