There are actually two correct answers here. The first correct answer is the previously given answer: the heart of the computer. The other correct answer is not given already in the answers above; instead, it is listed in the answer list. This is also called a central processing unit - also known as the CPU.
The CPU contains all the essential components to a computer - the motherboard, the hard drive, the various boards, etc. Everything that is labelled on the diagram pictured above is something that is needed to make a computer run. It’s kind of like the heart and the brains, but it’s often called the heart more often than the brains because it’s where the power comes from too.