This is the situation where heat spreads through a "conductor". That is, although not in direct contact with the heat source a body can be modify its thermal energy if there is heat conduction by another body, or another part of the same body.
For example, while cooking something, if we leave a spoon against the pan, which is over the fire, after a while it will heat up too.
This phenomenon happens because, as we heat the pan, its molecules begin to stir more, as the pan is in contact with the spoon, the molecules in greater agitation cause the molecules in the spoon to stir, causing their thermal energy to increase, soon her warming up.
It is also for this reason that although only the bottom of the pan is directly in contact with the fire, its top also gets hot.
Convection is the movement of fluids, and is the fundamental principle of understanding wind, for example.
The air in the plains is warmed by the sun and the ground, thus becoming lighter and rising. Then the air masses that are in the mountains, which are colder than the plains, take their place vacated by the heated air, and the heated mass moves to the highest places, where they cool. These movements cause, among other natural phenomena, the wind.
Formally, convection It is the phenomenon in which heat propagates through the movement of fluid masses of different densities.
It is the propagation of thermal energy that does not need a material medium to happen, as heat propagates through electromagnetic waves.
Imagine a microwave oven. This appliance heats the food without contact with it, and unlike the gas oven, it does not need to heat the air. While the food is heated there is a microwave emission that increases its thermal energy, increasing the temperature.
The body that emits radiant energy is called the emitter or radiator and the receiving body, the receiver.