Antenna efficiency

Antenna efficiency is about how much energy is put out by an antenna compared to how much energy the antenna is receiving. To think about it like a leaking bucket, if a bucket has a lot of holes in the bottom it won't be able to hold very much water, but if the bucket has just a few holes it will be harder for the water to leak out. That's what efficiency means - if an antenna has a lot of holes, or places where the energy is leaking out, it won't be as efficient as one with fewer places to leak.