Concentrate means to make something stronger, denser, or more focused. If you concentrate your energy, for example, you become more powerful! Concentrate your efforts at digging, and you'll plant that shrub a lot faster.
You may have seen the words "frozen concentrate" used to describe orange juice. That can of frozen orange goo is the juice in its purest, most condensed form — the concentrate — and it's much stronger and sweeter than the juice you'd drink. Another meaning of the verb concentrate refers to your mental focus. You might need to concentrate on what you're reading in order to understand it. When you concentrate, you strengthen your focus to shut out distractions.