How far apart two waves are, say in an ocean, is called the wave length. The farther apart they are, there is more space between them. That mean you see them that less often. So waves that are far apart will be happening less frequently. So higher wave length means low frequency. And lower wave length, or waves that are right next to each other, means higher frequency.

So a wave length and frequency are opposites and in a sense measure the same thing. How close waves are to each other.