~만 못하다’ means: worse(not better) than~
We use this korean grammar point when the first thing in the sentence is not as good as the last thing thing at the end of the sentence.
So something is less than you expected or wanted (My phone is good but my older phone was better). Or, just Thing A is better then Thing B. (Maria plays tennis better then James).
-가: 형제가 모두 운동을 잘하네요. (It looks like our younger brother all sports well)
나: 동생이 형만 못해요. (A동생