Consider two pictures of a wave. First plot displacement as a function of position - in other words, take a photograph, freezing the wave at one instant in time.

The wavelength is the distance between two particles that are 2p out of phase with one another. For instance, it's the distance between one peak and the next.

Now plot displacement as a function of time for a single particle. Here the "distance" between two peaks has units of time - it's the period of the wave.