What is the function of circuit breaker
?

As its name implies -- to break a circuit. It provides protection against overloads of the electrical line. If the current flow gets too high, the circuit breaker opens (or "breaks") the circuit, thereby shutting off the power for that line. This is done because if the current gets too high it can cause excessive heating of the wires, which could cause a fire hazard.

What is the situation you are confronted with that causes you to ask what a circuit breaker does?

Simple answer is in the words, it breaks a circuit.

Give more detail to your question so that you can get a more detailed answer.

It protects wire, and equipment.
With a #12 wire on a 20 amp breaker, an overload would cause breaker to trip, and not burn up wire.
Bigger wires need bigger breakers, However you may need #10 for a long run 120 volt
Outlet, you would still use 20 amp breaker