It is the movement of ions or molecules, mostly of gas from a region of higher concentration to a region of lower concentration. Just think of it this way..... If perfume is sprayed at one end of a room, and you're at the other end, you start to smell the perfume scent shortly later. This is because the perfume molecules have moved from a high concentration region(where it was sprayed) to a low concentration region which is the other end of the room. I hope I helped. Good luck.