Endpoints in OAuth 2 are targets with a specific responsibility and often associated with a particular URL. Because of this the word endpoint might be used interchangably from the endpoint url.
The main three responsibilities in an OAuth 2 flow is to authorize access to a certain users resources to a client, to supply said client with a token embodying this authorization and to verify that the token is valid when the client attempts to access the user resources on their behalf.
There are three different endpoints, the authorization endpoint which mainly handles user authorization, the token endpoint which provides tokens and the resource endpoint which provides access to protected resources. It is to the endpoints you will feed requests and get back an almost complete response. This process is simplified for you using a decorator such as the django one described later.
The main purpose of the endpoint in OAuthLib is to figure out which grant type or token to dispatch the request to.