HTTP is the most used protocol. For the past many years, browser was the most preferred client by which we consumed data exposed over HTTP. But as years passed by, client variety started spreading out. We had demand to consume data on HTTP from clients like mobile, JavaScript, Windows applications, etc.
For satisfying the broad range of clients REST was the proposed approach. You can read more about REST from the WCF chapter.
WebAPI is the technology by which you can expose data over HTTP following REST principles.