What does horsepower mean?

Horsepower is a measure of how many horses it would take to produce a given amount of power. In the 1780's Watt determined that a horse could walk around a 24ft diameter mill whell at 2.5 times/minute for a days work with a tractive effort of 180 pound force. this yields a power of about 33,000 ft- lb per minute.

Thus 1 Brake hp = 550 ft-lb per second.

also 1 hp = 746 Watts for electric motors.

It's a measure of how strong the engine is.

An engine that could pull a cart at the same rate as one horse would have been able to keep up the whole day is a one horsepower engine.