Not everyone can take a punch on the nose and brush it off as if nothing had happened. Few people can take getting shot or starved, tortured or wounded in the course of their everyday lives. However, people with the Tough trait are used to the adventure, can take the knocks and brush them off.
Tough reduces the amount of damage that would normally be deducted from the character’s attributes by 2. This is after any other effects, such as armour, are taken into account.
For example, Danny Pink had to face many hostiles when he was a soldier, which has made him more resilient. He fails during a Conflict with a Dalek, but not badly enough to kill him. He’d normally be told to knock 4 points off one or more of his attributes to reflect his injury, but his Tough trait reduces the damage by 2. Now he only has to take 2 points off his attributes. A wound all the same, but not as bad as it could have been.