An ice cube is 'cold' and a flame is 'hot'. Our bodies quickly tell us the difference between a day in summer and a day in winter and our mouths respond differently to a warm cup of tea and a hot cup of tea. A hot bar of iron is longer than a cold bar of iron and very cold water is a solid, where very hot water is gas. One property that changes with different degrees of temperature is the volume of liquids.
Hot liquids have larger volumes than cold liquids.
This property change gives us a way of measuring temperature. If a small amount of colored liquid is enclosed in a long thin tube so that it can expand and contract, we have a way of accurately determining the temperature surrounding the tube. As the surroundings heat up, the liquid in the tube expands and fills more of the tube. As the surroundings cool down, the liquid contracts and so the column of liquid in the tube will becomes shorter. Of course such a 'heat-measure' was invented a long time ago.