Toughness is the source of your Wounds and Stamina, the attribute that determines the amount of damage you can take before you go down.
Each Combat Keystone you take grants you between 3 and 5 levels of Toughness, the exact number depending on the Keystone.
When you calculate your Stamina and Wounds, you use your total levels of Toughness that you have received from all of your Keystones.
Your maximum Stamina is equal to your CON modifier plus 4 times the number of ranks of Toughness you have (which are granted by your Combat Keystone(s)).
Stamina = (4 x Toughness) + CON
Your maximum Wounds is equal to the total of your CON and STR modifiers multiplied by your Tier, plus 5 times the number of ranks of Toughness you have.
Wounds = (5 x Toughness) + (CON + STR) x Tier