This dispersion relation \(\omega(k)\) describes the relation between the frequency (energy) and the wavenumber (wavelength) of a monatomic chain of a crystal. The oscillation is purely longitudinal (or transversal) and only the interaction between the neighboring chains is considered here.
The angular frequency is related to the frequency \(f\) via \(\omega = 2\pi \, f \).
Wavenumber is related to wavelength \(\lambda\) via \(k = 2\pi / \lambda \).
Spring constant (or coupling constant) comes from the Hooke's law and describes how much an atomic chain is coupled to its neighboring chains.
Mass of an atom within the chain.
Lattice constant is the distance between two chains when they are in equilibrium (i.e. not deflected).