k is the parameter of the function as in the 'x'-axis so instead of saying y=f(x), the formula says y=f(k) You can see in the formula, that it's plugged in as

k is the parameter of the function as in the 'x'-axis so instead of saying y=f(x), the formula says y=f(k) You can see in the formula, that it's plugged in as the frequency of the signal the original signal is 'spun-around' with.