Fields are automatically given a default value when declared. If they are never assigned then they will always have their default value.

Check if an assignment to the field has been accidentally omitted. If it hasn't then consider removing the variable or explicitly initializing the field to its default value.

In this example the programmer has forgotten to initialize the "name" field. This is the kind of mistake that this rule can detect.

MSDN, C# Reference, Compiler Warning (level 4) CS0649.