A (Bronsted) Acid is a proton donor and its Conjugate Base is what remains of the acid once it donates the proton. In this case, CH3NH3I is the (Bronsted) Acid and CH3NH2 is the Conjugate Base because the Acid donated a proton (H+) to the Bronsted Base.. So in simpler words, CH3NH3I has one more 'H' than the CH3NH2.. and also remember that Buffers can't be made with either Strong Acid or Strong Base..
Hmm.. I'm sure there are others who can elaborate better, but I hope it helped!