You write the lyrics and your friend composes the music. Separately each of you is pretty good, but together you’ve got a mega-hit song. That’s synergy — working together to create something greater than either of you could do alone.
The word synergy comes from the Greek sun “together” and ergon “work” (the same root that gives us ergonomic and energy). When one thing magnifies the effect of another, together they have synergy. If your company makes some revolutionary kind of yo-yo, you may decide to acquire a company that distributes toys, as you’d expect there would be a natural synergy: each company will benefit from the other’s strengths, and together they’ll be stronger than either would be by itself.