Sharpness+1 extends the length of a weapon's sharpness. Sometimes it adds extra colour(say more Green Sharpness) to the bar or can give you an additional sharpness level (I.E Green to Blue) making the weapon much stronger both in raw and elemental. The trade offs being it's one of the hardest skills to work with as you need 2 slots for 1 point of Handcraft(10 for Sharpness+1)