The document discusses an open API for accessing city data from a database. It provides an example where a servlet uses the server application to call the getCityFor function, passing in a name and returning the corresponding city. It then returns the city in the response. The document also defines what an open API is, noting that it allows applications to make requests for specific data through protocols like REST. It explains that application developers must register their application with the open data provider to get an API key for authorization and can use security tokens for authentication of sessions.