Mantle conviction is the process in which heat is transferred from the Earth's core to its surface; heat is released from the core and rises, causing temperature fluctuation where the excess temperature from the hot magma is transferred to the colder areas above it and eventually to the Earth's surface. An everyday example of this process is boiling water; hot water from the bottom of the pot rises to transfer energy to the cooler water at the top, which sinks to the bottom.
Mantles in the Earth are solid but break down over millions of years into an unstable liquid form similar to silly putty or play dough. Convection can occur when the mantles are in this state; the heated semi-solid mantle weighs less as it warms, forcing it to rise through cooler rock, while the heavier, colder material is forced downward to repeat the process.
An extreme example of convection is volcanic eruptions. When the solid part of the Earth's crust turns into a semi-liquid, its forced upwards to be released as eruptions on the Earth's surface. Hot molten lava, rocks and boulders from the layers above and gas from the core fight their way to the surface and cool once released into the atmosphere.