Don't think I could properly explain brake horse power without confusing myself, so forgive me if I don't try.
But I think I can have a go at how it relates to horses....
It originates from when engines were first devised, they would be compared in pulling power by the ability of a single horse (which obviously preceeded engines!). It wasn't an exact science back then, just like measuring horses by hand-spans wasn't, but now it's been fine tuned and is now related to enrgy (kilowatts), but the old phrase has stuck.
Am I getting better at this??