Hydro Attack?

Hello i read the skill and says,

Attacks target with s1 Power added to M. Atk., and brands target with Storm Sign. 10% more damage if target is debuffed. Requires a sword or blunt weapon.

and what does it mean "s1 Power"?

Thx everybody

Now one knows what the fak Does it means S1 Power?

No clue what that means..