Convection is the process to transfer heat from one place to another by motion of the

particles. It occurs in a medium which has ability to flow, for example, fluids like liquid and gas. It is not possible in solids.
Temperature tells us the amount of interatomic vibrations between molecules or atoms of a material. High temperature means that there is a lot of interatomic motion and vibrations taking place and it leads to decrease in density of that material with increase in volume. Opposite happens at comparatively low temperatures. The interatomic vibrations are less and atoms or molecules are closer to each other and hence, the density is more.
Now when we talk about hot air and cold air, the cold air is denser than hot air. In a room containing a mixture of hot and cold air, the hot air being less dense rises while cold air sinks. This takes place through convection of air. If there are human bodies inside the room they will emit body heat into the air and if there isn't a source of cold air, the room will eventually heat up. So, in order to maintain convection currents to keep the air fresh and cool in the room, ventilators are provided at top.