What is a sub panel?

A sub-panel is always fed from a main panel. The main panel is situated where the electrical service wiring first enters the main structure on a dwelling plot or building site.

A sub-panel can be situated within the same building as the main panel or it can be in a subsidiary building or structure (such as a garage, garden shed or workshop) that is separate from the main building.

More information

A sub-panel is another name for a secondary breaker box, just as "the main panel" is another name for "the main breaker-box".

"Breaker-box" and/or "panel" are just alternative short names that are used instead of the full name "circuit breaker box".