A CPU is the Central Processing Unit-essentially the "brains" of the computer. Every computer has one. On this computer, the CPU is an AMD Ryzen 5, which is one of their new range.
When your friend says "a CPU", it sounds like he or she is talking about the tower in a traditional computer. Because this is an "all in one" unit, the computer itself is actually built into the back of the monitor, so a separate tower isn't needed.