It's a passive buff. Your physical damage is 4% higher (on skill lvl 1) when you use crossbow skills. When you put your mouse on it you see the description. Under the skill name it says either Passive or Active. Only the ones says Active are "usable" skills =)
Passive skills are like buffs that are activated all the time(you can`t cast them).
There are some guides about silkroad under "guides" section,feel free to browse them
it increase ur physical attack power of the xbow 
