In brief
HTML header groups introductory content such as headings, navigation aids or author information. It may belong to a particular article rather than only the entire site.
Useful to know
HTML header groups introductory content such as headings, navigation aids or author information. It may belong to a particular article rather than only the entire site. [1]
A document can therefore contain different header areas in suitable contexts. The name does not imply a sticky top bar; fixing it on screen is separate CSS behaviour.
Sources
For further detail and to check the information: