Circuit breakers get hot when the current exceeds the rating of the breaker. The electricity that flows through the circuit produces heat in the unit. When that heat reaches a certain point, the breaker trips and breaks the circuit. Warmth indicates that the circuit is at or near capacity.
Circuit breakers are designed to be the weakest point in the electrical circuit to prevent potentially dangerous overheating or faults within the walls, which could start fires. If a circuit breaker becomes warm to the touch, relocate some of the devices on that circuit to reduce the load on the current.