Broken in Mirrodin block, now playable in legacy and extended, but not the aggro deck of choice in either.
If this is a monster hunter question, it is the % chance of a critical strike, which deals 30% more damage. If the affinity is negative, it will do a feeble hit with 30% less damage.
building on the 2nd response, it's actually x1.25 damage if its s positive affinity strike (Red Slashes appearing when you attack) and x.75 damage if its a negative affinity strike (Blue Slashes appearing when you attack). So its +/- 25% damage.