If your home's service panel doesn't have room for new circuit breakers and you cannot use tandem breakers, a subpanel might be the answer. A subpanel connects to the main service panel with a thick three-wire cable. The feeder breaker in the main panel acts as the main disconnect for the subpanel. Before installing a subpanel, consult with an inspector to make sure you do not overload your overall system.
A subpanel has separate bus bars for neutral and ground wires and typically has no main breaker. It may not be labeled "subpanel." Instead, it might be labeled "lugs only." Additionally, it might be a different brand than the main panel. Have the inspector approve the subpanel, the feeder cable, and the feeder breaker before you start installing a subpanel.
Safety is most important when installing anything with wires or electrical work. When dealing with electrical projects, shut off the power first. Before you begin installing a subpanel, shut off the main breaker in the service panel.