OAuth is an open standard for authorization. It allows users to share private resources such as photos, videos, etc. stored on one site with another site. And this without handing out any credentials, typically supplying username and password tokens instead.
Each token grants access to a specific site (such as a video editing site) for specific resources (e.g: just videos from a specific album) and for a defined duration (e.g: the next 2 hours). This allows a user to grant a third party site access to their information stored with another service provider, without sharing their access permissions or the full extent of their data.