The Memory pane displays how much memory your Mac is using, how often it is swapping memory between RAM and your hard disk, and the amount of memory provided for an app and the how much of it is compressed memory.
When you have free or unused memory, your computer performance does not necessarily improve. OS X obtains the best performance by efficiently using and managing all of your Mac’s memory.
Click Memory in the Activity Monitor window.
The Memory Pressure graph lets you know if your computer is efficiently using your memory.
Green memory pressure: Your computer is efficiently using all of your RAM.
Yellow memory pressure: Your computer might need more RAM eventually.
Red memory pressure: Your computer needs more RAM.
If your memory pressure is yellow, red, or has spikes, check to see if an app is using up the memory and causing the memory pressure to increase.
Your computer’s memory pressure is accurately measured by examining the amount of free memory available, the swap rate, and the amount of wired and file cached memory to determine if your computer is efficiently using your RAM.
To learn how to add more RAM to your Mac, choose Apple menu > About This Mac, click Memory, then click Memory Upgrade Instructions.