In a sine wave, the wavelength is the distance between peaks:
The x axis represents distance, and I would be some varying quantity (for instance air pressure for a sound wave or strength of the electric or magnetic field for light), at a given point in time as a function of x.
Wavelength has an inverse relationship frequency, the number of peaks to pass a point in a given time. The wavelength is equal to the speed of the wave divided by the frequency of the wave. When dealing with electromagnetic radiation in a vacuum, this speed is the speed of light c, so the conversion becomes,
For radio waves this relationship is easily handled with this formula: meters of wavelength = 300/frequency in megahertz (MHz)