What's the "Content-Length" field in HTTP header?

It's the number of bytes of data in the body of the request or response. The body is the part that comes after the blank line below the headers.

rfc2616

The Content-Length entity-header field indicates the size of the entity-body, in decimal number of OCTETs, sent to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.

It doesn't matter what the content-type is.

Extension at post below.

The Content-Length header is a number denoting an the exact byte length of the HTTP body. The HTTP body starts immediately after the first empty line that is found after the start-line and headers.

Generally the Content-Length header is used for HTTP 1.1 so that the receiving party knows when the current response^* has finished, so the connection can be reused for another request.

_{^{* ...or request, in the case of request methods that have a body, such as POST, PUT or PATCH}}

Alternatively, Content-Length header can be omitted and a chunked Transfer-Encoding header can be used.

If both Content-Length and Transfer-Encoding headers are missing, then at the end of the response the connection must be closed.

The following resource is a guide that I found very useful when learning about HTTP:

HTTP Made Really Easy.