The exact meaning of the term depends on context.
Certain frameworks to describe IT service management use the term service description; for instance, it may be a standard part of a service level agreement.
[1] In software systems with a service-oriented architecture, certain parts of the software act as services that other parts of the software invoke by sending a request and receiving a response in return.
In this context, a service description describes the structure of the request and of the response.
The exact way to do this depends on the standard being used.