Primary memory is computer memory that is accessed directly by the CPU. This includes several types of memory, such as the processor cache and system ROM. However, in most cases, primary memory refers to system RAM.
RAM, or random access memory, consists of one or more memory modules that temporarily store data while a computer is running. RAM is volatile memory, meaning it is erased when the power is turned off. Therefore, each time you start up your computer, the operating system must be loaded from secondary memory (such as a hard drive) into the primary memory, or RAM. Similarly, whenever you launch an application on your computer, it is loaded into RAM.
The operating system and applications are loaded into primary memory, since RAM can be accessed much faster than storage devices. In fact, the data can be transferred between CPU and RAM more than a hundred times faster than between the CPU and the hard drive. By loading data into RAM, programs can run significantly faster and are much more responsive than if than constantly accessed data from secondary memory.
NOTE: Primary memory may be called "primary storage" as well. However, this term is somewhat more ambiguous since, depending on the context, primary storage may also refer to internal storage devices, such as internal hard drives.