Are they the temperatures at which the NH3 begins to boil?
Yes, it is the temp at which ammonia begins to biol off. Note that this is not the boiling point of pure ammonia. As has been said, as the ammonia boils off the bp of the solution will rise gradually until all the ammonia is gone and you are left with pure water boiling at 100°C.
what does the melting point refer to? Does this refer to the melting point of a block of ice that 25% NH3 by weight as in impurity?
Yes. It means that if you take an aqueous solution that is 25% ammonia by weight and gradually cool it down, it will freeze at -57.5°C