Toughness is a rating of a character or creature's resilience.
Toughness' main roll lies in using it to calculate a characters starting life, simply take the characters base Toughness and multiply it by 10. Note that life has no maximum and can be increased with spells. If a characters life reaches 0, they die.
If a characters toughness is higher than their opponents Power, they may attempt to resist Physical Damage from them. If you choose to do so, both characters roll a d6, adding that roll to their tested scores (the attackers power, the defenders toughness). If the defender's modified score is higher, they resist damage equal to the difference of the two modified scores. If the attacker's modified score is higher, they deal extra damage equal to the difference of the two modified scores.