A request header is an HTTP header sent by the browser, along with the HTTP request.
The request header lets the browser pass information related to the HTTP request, such as what the browser wants, what file formats are accepted by the browser, what type of compression is accepted by the browser, etc.
Click on a request on the Waterfall Chart to view its Request headers.
The request header also contains information related to the version, type, and capabilities of the browser making the request so that the server can send compatible data.