Convection is the transfer of heat from a warmer region to a cooler one by moving warm liquid or gas from the heated area to the unheated area. In a boiling pot of water, hot water in the bottom of the pot rises to the surface, leading to bubbles of heated water and sometimes steam visible on the surface. In meteorology, convection is often associated with rising air and clouds, and at times, thunderstorms. Air that is rising cools as it reaches lower pressures, and may reach the point where water vapor in the air condenses and forms clouds. These rising columns of air are called "thermals". Fair weather cumulus clouds often form on the top of these rising columns of air. Thunderstorms can sometimes form where there is a lot of water vapor and heating. You may have heard the TV meteorologist mention afternoon convection leading to thunderstorms on hot summer afternoons.
Convection is not just limited to meteorology. In fact, many of our daily activities involve or observe convection. For example, if you drink coffee (or any hot beverage), you may notice steam rising up from your hot beverage. In this case, we can visibly observe convection as the steam is transferring heat into the air. Another example would be a heater during the winter. The heater emits warm air which will rise to the top of the room. The warm air will eventually cool and sink to the bottom of the room before getting pulled back into the heater. Over time, this process will warm and mix the air as long as the heater remains on.