What is oil thermal expansion in the CFM LEAP 1A engine for the A320NEO?
When oil heats up, its volume expands (and its density goes down). This means that if you measure the level of oil in a gearbox when it is hot, the level will read high, and if you come back later and measure it after it has cooled back down to ambient, it will read low.

So if you were to fill a sealed gearbox with cold oil without taking its thermal expansion into account, when it then gets hot it might expand enough to overpressurize the gearbox and blow out its seals. 

I had a tractor engine in which overfilling it with cold oil would then cause it to pump the box containing its ignition points full of engine oil once the engine had gotten hot. This would then foul the points and stop the engine.