What is thermal contraction?
Thermal contraction is the "shrinking" of substances as they cool.
Changes in temperature change the physical properties of substances- each substance does this on a different level, but with changes in temperature, substances either expand or contract. This is due to their energy levels on an atomic level.
When something is being cooled, it is losing heat energy- You can think of it as the atoms getting less and less excited. Therefore, the atoms start moving slower and closer as they lose heat energy.
Thermal contraction is therefore the length/size/volume of a substance decreasing as it gets cooled. It is the opposite of thermal expansion, in which those properties increase as a substance gains heat.