Horsepower (hp) is a unit of (naturally) power, or energy delivered per unit time. It is most commonly used to rate the power delivered by internal combustion engines (automobiles, lawn mowers). For a precise definition and history of this unit, this entry in Wikipedia is a good read. There is some variability in the precise conversion between horsepower and other units, but it is approximately equal to 745 watts.
I find this value rather interesting, as this is roughly the incident power delivered, per square meter, of radiation from the sun -- and also roughly the area available on a horse for collecting said power. And of course, harvesting that power is a rather inefficient process -- about 10-20% for solar cells.
Something to think about when you see an automobile rated at 200 hp: it would take 2000 square meters of (cheap) solar panels to deliver the equivalent power. That is nearly half of a football field. Try towing that behind your car.