When a substance is heated, the internal energy as well as vibrations of its molecules increases due to absorption of heat. Due to vibrations, the molecules in the substance also collide continuously with their neighbouring molecules and exert pressure on each other. Due to internal collisions, the whole substance experiences an outward pressure, and the separation between molecules of the substance starts increasing. This is called thermal expansion of body. For basic understanding of thermal expansion, watch the video below.
The internal energy of molecules is measured by temperature of the substance on different scales like Celsius, Kelvin, Fahrenheit, Reaumur, etc. For measurement of temperature on a scale, two fixed points are defined which are corresponding to freezing and boiling point of water. To understand the scales and conversions watch the video below.