One of the biggest issues “switchers” have when first using a Mac is knowing what’s going on. Macs lack drive lights, so you can’t see when they’re accessing the disk, for example.
One of the most popular apps for the Mac is Menu Meters, which adds various graphs and displays to the menu bar. However, a similar feature is already built-in to OS X.
Start Activity Monitor — it’s in the Utilities folder within the Applications list of Finder. This will open a window showing the programs and processes running on your system, but you can close it. Instead, right-click Activity Monitor’s Dock Icon, then select Dock Icon and choose from one of the choices — showing CPU, network, disk, or memory usage. You can also select CPU History, which will show a constantly updating vertical bar graph showing CPU load.
If you choose one of the usage options, you’ll see a chart showing the current state of affairs. The color key is as follows:
CPU usage (graph):
Green=Load from user’s apps
Red=Load from system tasks/apps
Memory usage (pie chart):
Green=Memory not being used
Red=Memory that can’t be swapped out to disk
Yellow=Memory being used
Blue=Memory recently freed-up
Disk usage (graph):
Network usage (graph):