When something is reinforced, it is stronger or more supported than before, like a reinforced attitude that is stronger because peers and family members also have it, or a reinforced floor that has extra pillars and beams holding it up from below.
Reinforced is the adjective form of reinforce, a verb that means "to strengthen." So a good synonym for reinforced is strengthened. In high-traffic areas like school floors and stairwells, builders choose reinforced concrete, which has an internal structure of metal bars, because it will last longer. Your socks might have reinforced toes, meaning thicker fabric in that part, to prevent holes from forming.