The Server header describes the software used by the origin server that handled the request — that is, the server that generated the response.
Warning: Avoid overly-detailed Server values, as they can reveal information that may make it (slightly) easier for attackers to exploit known security holes.
A name of the software or the product that handled the request. Usually in a format similar to User-Agent.
How much detail to include is an interesting balance to strike; exposing the OS version is probably a bad idea, as mentioned in the earlier warning about overly-detailed values. However, exposed Apache versions helped browsers to work around a bug of the versions with Content-Encoding and Range in combination.