This is the free memory currently available to ScriptThing.  Over time, memory may become fragmented, which may cause ScriptThing to run low on memory sooner than this number would indicate.

This is the total memory assigned to ScriptThing.  To give it more memory, quit ScriptThing, select its icon in the Finder, choose Get Info from the File menu, and change the Preferred Size to a larger number.

This bar shows the amount of memory currently in use, as a fraction of the total memory assigned to the application.  The left side of the bar represents memory in use.