Well you always miss on an 1, and the 'attack roll' for grenades it to get everyone effectively within the AOE. Then they get a saving throw (varies) to half or no affect, etc. If you 'miss' then the grenade deviates some distance, depending on your attack roll. If targets are still within the AOE they gain advantage on their save. If you roll a natural 20 on your attack roll then targets are at disadvantage to their saves.