Hello,
RPM stands for Revolutions per minute in terms of a hard disk (a some other products too)
In short, it tells you the access speed of the hard disk.
In detail:
It is used to help determine the access time on computer hard drives. RPM is a measurement of how many complete revolutions a computer's hard drive makes in a single minute. The higher the RPM, the faster the data will be accessed; for example, if you were comparing two hard drives, one with 5400 RPM and another with 7200 RPM, the hard drive with a 7200 RPM will be capable of accessing data much faster than the 5400 RPM drive. However, it is also important to note that a 5400 RPM drive will be much cheaper than a 7200 RPM drive.
While it may not be a noticeable speed difference when loading small files, the RPM of a hard drive can make a dramatic difference when loading large files or several hundred or thousand files. We always recommend users get at least a 7200 RPM drive when buying a traditional hard disk drive.