If you look in your basement or maybe the garage, you should find a large panel. This is the main distribution of electricity (ac mains) throughout your house. This panel will consists of switches called "breakers".
Each of these breakers represent one circuit or a branch.
This circuit has a maximum rating of how much can be loaded. Usually 15
amps for a standard circuit. Each circuit will be wired to a group of lights, electrical outlets, switches, etc.
Every device that requires ac mains to operate, is considered a "load". Each of these loads take a certain amount of "amps". For example, lights, TV, radio, computer.
If you connected all these devices to same circuit and turn them all on, then the "amps" they consume adds up. If you connect too many, then you may "overload" it because you exceed 15 amps, thereby causing the breaker to "break", cutting electricity to this circuit.
Another explanation here.