What's the difference between "Inactive" and "Free" memory

Inactive memory is memory that has been allocated, or mapped, to a process or hardware, but is not currently in use by its owner. Free memory is unallocated memory, available to any process, as needed.

 

You can freely use inactive memory if necessary.

OS X tries to put things you probably need soon in the inactive memory space so you can quickly draw stuff from memory rather than your HDD. But when you run memory intensive apps, do multi-tasks, etc., inactive memory will be treated pretty much the same as free memory by emptying the I-though-you'd-use-these-soon-but-you-didn't kind of stuff.

So in a sense, free memory + inactive memory = memory you can use freely. If you have larger inactive memory at the moment, that means OS X is working hard to make your MBP run faster while functionally keeping the same amount of freely available memory.