Stamina is an important stat because it's what you need to use a skill. The ammount of stamina you have is determined by willpower . There is some confusion to people with the terms energy, mana and stamina These are all one in the same. Mana is more generally used to refer to mages and energy for the warriors and rogues. 

It's not needed to invested in to much but early on for new players this can be a problem when it runs out. Investing in Stamina stealing is advised for Combat Masteries .

Also since new players will often run out of this there is 3 points gained per tick to help. 

Poison is the nerf to stam. It will drain away the ammount of poison and then half each time after that.