1.3.4. GetFeatureInfo (optional Attribute)

"GetFeatureInfo is an optional operation. It is only supported for those layers for which the attribute queryable="1" (true) has been defined or inherited. A client shall not issue a GetFeatureInfo request for other layers. A WMS shall respond with a properly formatted service exception (XML) response (code = OperationNotSupported) if it receives a GetFeatureInfo request but does not support it.
The GetFeatureInfo operation is designed to provide clients of a WMS with more information about features in the pictures of maps that were returned by previous Map requests. The canonical use case for GetFeatureInfo is that a user sees the response of a Map request and chooses a point (I,J) on that map for which to obtain more information. The basic operation provides the ability for a client to specify which pixel is being asked about, which layer(s) should be investigated, and what format the information should be returned in." (OGC 2004)
If you want to learn more about the GetFeatureInfo operation, have a look at the following pdf file (pages 31-33):OGC Web Map Service Interface