I think that's referring to disk space available on your hard drive.
The memory I'm talking about should be from 2Gb to 8Gb on a new machine, it's basically random access memory (on memory chips) so the computer is not dependent heavily on the disk page file (slow) system,(this is a basic, basic explanation). I would also recommend that you close all windows, Internet, games, etc. etc. and let Malwarebytes run by itself. Think that's how they recommend running it.
Installed memory: 3.00 GB (2.74 GB usable).