It means what is says - a serializable class should declare a serialVersionUID field. This filed is used to ensure that the serialized version of an object matches the current class implementation. Concrete example: class has fields aaa and bbb, you serialize an instance of the class, then change the class to have fields aaa, bb and ccc. What happens if you try to deserialize the earlier object that contains values for only aaa and bbb? By keeping an up-to-date serialVersionUID field the serialization code can tell you that your objects are mismatched.
To solve it, use Eclipse's auto-correct facility: with your cursor on that line click CTRL-1 and choose the option to generate a serialVersionUID.