cos(ωt-βz) is the term representing phase of a transverse electromagnetic wave in free space, where, ω is angular frequency (rad/s), β is phase constant (rad/m)
Note: Here we are using the convention that wave propagation is in z-direction.
We can see in the visualization that for the variation of time, in one time period the wave moves by one unit lambda (2π radians in space) and at a given point in space the point travels 2π radians in time.
We can also see that the equation can be broken down into a simpler basic equation using the fact that ω = 2πf and β = 2π/λ
c = ω/β = 2πf/(2π/λ) = (2π/T)/(2π/λ)
Hence, c = fλ