Consider two pictures of a wave. First plot y as a function of x, set the wave going and then hit the pause button. This is like taking a photograph, freezing the wave at one instant in time.

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

In our other view we focus on the motion of a single particle, plotting y as a function of time. Here the "distance" between two peaks has units of time - it's the period of the wave.