The phrase "additional boosters" means extra things that help to improve or increase something, especially when it is already good or effective. Full definition
Puppies may require additional booster vaccines depending on when they are adopted. (savealabrescue.org)
Your inventory can also be upgraded to hold more weapons at once as well as additional booster items that are earned by completing optional objectives in each mission. (darkstation.com)
The veterinarian gives additional boosters every two to four weeks until the puppy is 16 weeks old. (dogcare.dailypuppy.com)