Is the amount of RAM allotted to different programs and applications. The amount of allocated memory is preset when you install your program and varies depending on how large the application is.
Making sure that your computer has enough RAM will 1.) increase the ease at which the program processes the file 2.) cause less strain on your system and 3.) prevent your system from crashing.
On a Macintosh, you can change the allotted amount of RAM to a given
program by clicking once on the program icon, choosing "get info" from
the file menu, and changing the minimum and preferred sizes in the
memory requirement box. You must click the actual icon; it won't work with an alias, and you must be closed out of the program. This option is beneficial when working with large applications, lets say in Photoshop, where an increase of RAM is needed when working on those 50+ MB documents.
On the Windows platform, allocated memory cannot be changed manually. Windows tries to determine how much RAM will be needed for a given application.