A SQL Server endpoint is the point of entry into SQL Server. It is implemented as a database object that defines the ways and means in which SQL Server may communicate over the network. 

SQL Server routes all interactions with the network via endpoints and each endpoint supports a specific type of communication.

To participate in Always On availability groups or database mirroring, a server instance requires its own, dedicated database mirroring endpoint.

https://docs.microsoft.com/en-us/sql/relational-databases/server-management-objects-smo/tasks/implementing-endpoints?view=sql-server-ver15