A postback is a request sent from a client to server from the same page user is already working with.
ASP.NET was introduced with a mechanism to post an HTTP POST request back to the same page. It's basically posting a complete page back to server (i.e. sending all of its data) on same page. So, the whole page is refreshed.
Another concept related to this approach is "Callback" that is also asked sometimes during a technical interview question. Click here to understand Postback Vs Callback in ASP.NET.