A sub-panel is a smaller electrical panel that services a specific area of the home. It holds separate circuit breakers from the main breaker panel. It's usually located near the area it powers. Sub-panels are best when multiple circuits are needed in a single area separate from your main house, such as a workshop or greenhouse.
Sub-panels get electricity from a circuit in the main panel and do not have their own disconnect. The amp rating of the circuit in the main breaker panel must be the same or less than the rating of its connected sub-panel. Also, the only limit for the number of sub-panels you can have is the number of available circuits in your main breaker panel.