there is a slip mechanism called a torque converter, that at low speeds, works like a fluid coupling. that's why the vehicle creeps forwards when you ease off the brake.

think of a fluid filled donut with an impeller driven by the engine, and another impeller driving the transmission. at idle speeds the system has some slip. at higher speed differential there is torque multiplication due to a stator inside redirecting fluid flow. but that's well beyond ELI5 without diagrams and videos.

manual transmission cars don't have a fluid coupling, but rely on a friction clutch that works much like a brake pedal but inversely. stepping on the pedal releases the clutch into false neutral. and releasing the pedal engages the clutch and lets you bark the tires and get going.