It is important that you know how much memory is available on your computer. The Finder program has a command, under the Apple menu, that tells you this information, along with an accounting of memory use by active programs. Each program on the computer carries information that is seen by the Finder, telling it how much memory it needs to function, both a minimum amount and a recommended amount. The Finder allows a program to run if there is a minimum amount available, but not the amount recommended, but only after receiving approval from you. The suggested size shown in the Get Info window is usually the minimum amount of memory needed to run the program.
Some programs run better or can handle larger and more complicated files if given more memory.
As was mentioned in the section about running multiple programs, you can run only a certain number of programs in the computer at the same time. This is limited to size required by each program and the amount of memory available. The best way to make sure you have enough memory in a limited environment is to quit a program when you are finished before beginning another program. Disk storage space is a different issue. Each disk has a fixed size.