What Does Hard Drive RPM Mean?
RPM stands for "revolutions per minute". A standard hard drive has a spinning disc inside of it - RPM measures how many times that disc spins in a minute. As RPM increases, the speed of your hard drive also increases. So a hard drive with a higher RPM represents a faster hard drive. An iMac, for example, might come with an option for a 5400 RPM hard drive or a faster 7200 RPM option.
Most Mac hard drives are SSD (solid state drives), which do not have any moving parts, and thus do not have an RPM rate associated with them.