Question

what is meant by air expanding on heating

Answer 1

The temperature of a gas is a measure of the speed of its molecules. ... When the bottle is heated, the air molecules inside start moving faster. These molecules now collide into the balloon with more energy resulting in increased pressure. The increased pressure causes the balloon to expand.

Answer 2

it means for something to be hot is that all the little parts are rattling around intensely. For a gas, that means that the fast-moving molecules will bang into things often and hard. In other words, the gas will exert more pressure on its surroundings. Often, what happens is that the gas pushes things (say balloon walls or neighboring gas) out. That leaves more room for the hot gas. The pressure drops, partly because the expanded gas just doesn't have as high a density of molecules bumping into things.