Go outside your house or apartment. Look at the electrical power cable that runs into your home. Notice that as the wire nears your home, it enters into some sort of hard housing (metal or plastic). The part of the housing where the cable enters is called the weatherhead.
It's designed to prevent water and stuff from dripping down on the cable into your home. The whole thing should be at least partially protected from the elements by your roof or something.
Sometimes, the cable has a droop in it before it enters the weatherhead, in order to encourage water to drip to the bottom of the droop instead of the weatherhead.
If you open your circuit breaker panel, and find water or rust, you should have your weatherhead examined. This is a bit outside your regular DIY home improvement project, as it involves electricity straight from the transmission wires.