Depolarisation is a change in the potential difference across the membrane
if you are refering to depolarisation of axon membrane during an action potential then upon the arrival of an action potential there is a change in the permeability of axon membrane to sodium ions , sodium ion channels open allowing the influx of Na+ ions into the membrane this will make the inside more positive as compared to the outside the membrane is depolarised there is a difference in potential difference across the membrane.
Thats it hope its clear, there might be some mistakes as i am also doing the exam this june
Thank-you very much****************
