The Kb value is an indicator which tells how strong is a base. The homologue for the acids is the Ka.
When a base is completely dissociated in the medium, it is considered a strong base and its Kb is going to be very high. For example, for the NaOH the Kb is 0.63.
4.4*x10-4 is a very low number compared to 0.63. It means that the CH3NH2 is a weak base.