This Booster temporarily increases the player's endurance, allowing them to take more punishment than usual. Combat scenarios like boss fights will be much easier with Toughness. It does not, however, affect the player's health gauge.
Think of Toughness like putting on an armor vest. Players will absorb more damage when using it; this includes hits from firearms, melee weapons, and zombie attacks. Explosive damage is also somewhat reduced. Should a player be expecting trouble, Toughness will come in handy.
This Booster's primary uses are for bosses, situations in which players are cornered by deadly foes, and Quarantine Zone missions. Use it to survive intense combat and boss fights; it will make a sizeable difference and reduce the need for Med Kit usage. But Toughness alone won't be enough to win the fight; strong weapons and/or other Boosters are recommended, such as Stamina and Speed.