Random Access Memory (RAM) being installed into the system is not enough for the system to boot. On top of the RAM we need a kernel to manage it. This means allocating RAM, or memory, to programs (processes/threads) that you run, enabling them to store information.
The kernel also takes care of "cleaning up" memory that is no longer used by a program so that it can be used again by another.
Because most modern, advanced computers in this day and age run multiple processes, and each process can allocate (ask for) its own memory (RAM), it's extremely important that the kernel manages the memory usage of each process and handle their requests for more memory or to clean up memory.
Technically memory management falls under the follow section on general resource management, but RAM/memory is one of those topics you should pay a bit of special attention to. This is because RAM is an expensive, finite resource and your ability to identify when a system is running low or an application is using too much is an important part of managing infrastructure.