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Inactive memory gets a horrible rap due to a crappy name. It should be called "make your Mac really fast the second, third, and fourth time it does the same task" memory except that's an awful name, too.

Here's how I explain things to someone new to the concept of virtual memory:

Inactive memory got a horrible rap due to a crappy name. It should have been called something like "make your Mac really fast the second, third, and fourth time it does the same task" memory except that's an awful name, too.

Apple re-wrote the activity monitor when it introduced several new features and added compressed memory, so some of this no longer applies to macOS that don’t show “Inactive Memory” but this still applies to Mac OS X memory management as documented against this version of Activity Monitor: https://support.apple.com/en-us/HT201538

Here's how I explained things to someone new to the concept of virtual memory on OS X:

• Wired: The system cannot run without this amount of RAM (never swapped)
• Active: Programs are really using this memory now or in the last few seconds
• Inactive: Things that programs have read from the slow disk or elsewhere and said they never need again. Engineers know better, you will go back to Facebook in a few minutes or re-launch Word after quitting it. The same things happen again and again on computers.
• Free: Totally Wasted RAM - the system only needs one or two MB free to cover short term allocation requests. For largest allocations, it simply uses some of the Inactive RAM by allocating it to active/wired and removing the information on what it used to hold.

• Wired: The system cannot run without this amount of RAM (never swapped)
• Active: Programs are really using this memory now or in the last few seconds
• Inactive: Things that programs have read from the slow disk or elsewhere and said they never need again. Engineers know better, you will go back to Facebook in a few minutes or re-launch Word after quitting it. The same things happen again and again on computers.
• Free: Totally Wasted RAM - the system only needs one or two MB free to cover short term allocation requests. For largest allocations, it simply uses some of the Inactive RAM by allocating it to active/wired and removing the information on what it used to hold.

• Wired: The system cannot run without this amount of RAM (never swapped)
• Active: Programs are really using this memory now or in the last few seconds
• Inactive: Things that programs have read from the slow disk or elsewhere and said they never need again. We know better, you will go back to Facebook in a few minutes or re-launch Word after quitting it.
• Free: Wasted RAM - you only need one or two MB free since we can simply use some of the Inactive RAM if you need more RAM to do something.

• Wired: The system cannot run without this amount of RAM (never swapped)
• Active: Programs are really using this memory now or in the last few seconds
• Inactive: Things that programs have read from the slow disk or elsewhere and said they never need again. Engineers know better, you will go back to Facebook in a few minutes or re-launch Word after quitting it. The same things happen again and again on computers.
• Free: Totally Wasted RAM - the system only needs one or two MB free to cover short term allocation requests. For largest allocations, it simply uses some of the Inactive RAM by allocating it to active/wired and removing the information on what it used to hold.