The de Broglie wavelength of a particle is a wavelength based on its momentum. The particle's wavelength and momentum are related in the same manner as a photon's. For either:
λ = h/p
Louis de Broglie explored the idea that particles might follow rules analogous to photons, and the consequent predictions are confirmed by experiment, e.g., the behavior of an electron moving toward a slit differs regarding whether another (parallel) slit is nearby, exhibiting the same sort of behavior as does a photon, i.e., showing wave-like interference.
This is a key concept in quantum mechanics, the odd type of mechanics that successfully describes the behavior of small particles. The supposed waves are known as matter waves or de Broglie waves, their frequency, the de Broglie frequency.