Fields are the ways you get data values in graphql and a field definition represents a field, its type, the arguments it takes
and the DataFetcher used to get data values for that field.
Fields can be thought of as functions in graphql, they have a name, take defined arguments and return a value.
Fields can also be deprecated, which indicates the consumers that a field wont be supported in the future.
See http://graphql.org/learn/queries/#fields for more details on the concept.