more soldiers, police etc who are sent to a battle, fight etc to make their group stronger:
The police called for reinforcements.
positive reinforcement is when you give someone praise or rewards for their behaviour or work, so they want to continue doing well. Negative reinforcement is when you give someone punishments or criticism when their behaviour or work is bad, so that they want to improve to avoid punishments again:
We need to give students plenty of positive reinforcement.
the act of making something stronger:
The bridge needs some structural reinforcement.