What is an Endpoint?

All of the answers posted so far are correct, an endpoint is simply one end of a communication channel. In the case of OAuth, there are three endpoints you need to be concerned with:

1. Temporary Credential Request URI (called the Request Token URL in the OAuth 1.0a community spec). This is a URI that you send a request to in order to obtain an unauthorized Request Token from the server / service provider.
2. Resource Owner Authorization URI (called the User Authorization URL in the OAuth 1.0a community spec). This is a URI that you direct the user to to authorize a Request Token obtained from the Temporary Credential Request URI.
3. Token Request URI (called the Access Token URL in the OAuth 1.0a community spec). This is a URI that you send a request to in order to exchange an authorized Request Token for an Access Token which can then be used to obtain access to a Protected Resource.

Hope that helps clear things up. Have fun learning about OAuth! Post more questions if you run into any difficulties implementing an OAuth client.