To reinforce means to make stronger. You can reinforce your roof by putting some extra supports under it.
Besides a physical structure, there are many things you can reinforce. If you want to reinforce a message, you can say it louder, rephrase and repeat it or give examples of what you expect. You use dog biscuits to reinforce certain behaviors in your dog. Bad experiences will reinforce your fears, good ones your hopes. And when military leaders reinforce their troops, they send additional soldiers to the front.