In a wave motion $y = a \sin (kx - \omega t)$, $y$ can represent
In a wave motion $y = a \sin (kx - \omega t)$, $y$ can represent
velocity
acceleration
Displacement
All of these
Solution
In case of sound wave, $y$ can represent pressure and displacement, while in case of an electromagnetic wave, it represents electric and magnetic fields.
In general, $y$ is any general physical quantity which is made to oscillate at one place and these oscillations are propagated to other places also.