The access modifier override is required when a field is declared which also exists on a parent class. Its purpose is to ensure that the author of a class is aware of the override as this may not always be obvious in large class hierarchies. Likewise, having override on a field which does not actually override anything (e.g. due to a misspelt field name) triggers an error.
The effects of overriding fields are detailed in Overriding Methods. This modifier is only allowed on method fields.