Power line adapters carry the ethernet signal through the electrical cable. Instead of trying to reach the outbuilding with wifi, you carry the signal from the house router to an A/C outlet in your home through the cable to the outbuilding. There is an adapter on each end. At the outbuilding you install a wireless Access point and plug it into the adapter ethernet port.
The important part is, the house adapter must be plugged into a receptacle on the the same circuit as the outbuilding. If the outbuilding is fed directly from the utility company, this won’t work. If it has a sub panel fed from the house, it could. Note that when I say “circuit” I’m referring to the breaker panel feeds. Odd/Even #breakers are on different legs, so if a sub panel is fed from an “even” breaker for example, the adapter would also have to be fed by an “even” breaker. Check it out on the web for more details.