In a wave motion $\mathrm{y}=\mathrm{asin}(\mathrm{kx}-\mathrm{\omega t})$, y can represent

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 , in  general , disturbance and this disturbance propagates to other places through the intervening medium.)