This term has transferred to web design from newspaper publishing.

It refers to the common elements usually found at the top of a web site.

Most web sites follow fairly standard design patterns in order to make it easy for people to know how to use their web site.

A masthead usually contains elements such as a logo or other branding picture, some navigation elements and perhaps prominent contact details such as a telephone number.

The important thing is that the masthead remains the same on all the pages.