The leagues endpoint is for most customers the starting point. A request on this endpoint would return a response with all Leagues you have access to, based on the plan you are subscribed to.
The leagues endpoint provides you League information like its id, name, country, coverage etc.
The endpoint is often used in combination with the season or seasons includes. Where the season include returns information about the active season only, the seasons Include returns information about all seasons available for the specific Leagues.