While Lovesong is correct in that the term "memory" usually relates to RAM, it can and often does mean physical drive space when it comes to those pesky warnings from your Mac.
It's a general rule that you need to keep a minimum of 10% of your boot drive as free space - the Mac OS uses some of that space as virtual memory and for swap files. When drive space gets too low, things can start getting wacky.
It's also a good practice to maintain at least 10% free space on any other drive that is used for editing purposes (captures, renders, exports, etc).
-DH