The judge is talking about the IVs of your Metagross(pwns). When he says "This Pokemon's potential is above average overall", he means the total amount of IVs your Metagross has and apparently your total is between 91-120 which is not bad.
When he says "Incidentally, i say that It's best potential lies in its Attack stat", he is say which stat has the highest IVs overall. So your Metagross Attack stat has the highest amount of IVs out of all the stats.
When he says "It cant be better in that regard. That's how I judged it", he Is saying the amount of IVs that are in your Attack stat. Apparently, your Metagross has 31 Attack IVs in the stat which is amazing. Look at the link for more Info.
Edit:"(The first statement) If the IV total is between 0 and 90: "This Pokémon's potential is decent all around." If the IV total is between 91 and 120: "This Pokémon's potential is above average overall." If the IV total is between 121 and 150: "This Pokémon has relatively superior potential overall." If the IV total is between 151 and 186: "This Pokémon has outstanding potential overall."(The second statement) If the highest IV is between 0 and 15: "It's rather decent in that regard." If the highest IV is between 16 and 25: "It's very good in that regard." If the highest IV is between 26 and 30:"It's fantastic in that regard." If the highest IV is 31: "It can't be better in that regard."
Those are all possible comments found in that link.