Convection is the transfer of heat energy by means of a fluid (liquid or gas) through bulk movement of the fluid itself. When a fluid is heated it expands (particles take more space due to increased movement) which makes it less dense, so the hot fluid rises passes the cold fluid. This sets up a convection current. Convection currents can form around any object which are cooler or warmer than its surroundings. E.g. energy from a cup of tea will be transferred to the air around it making less dense and rise up. Cooler air takes its place and
this cools the tea down. Convection currents also form around cold objects. Energy is transferred from the air to the cold object so the air cools down. The air becomes more dense and sinks. Warmer air takes its place.
A convection current is the movement of fluid caused by the change in densities in various parts of a fluid.