Theory:

Heat is a form of energy transferred between two objects, materials and systems with different temperatures, generally from higher to lower temperatures. It is also known as Heat energy or Thermal energy.
It is one of the essential energies for daily life.
 
Examples of daily activities using heat energy are,
  • Cooking
cooking-on-a-fire-5175531_1280.jpg
Burning clay stove 
  • Ironing
A_Local_Taylor_ironing_cloth.jpg
Iron box
  • Transportation
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Rail engine
Heat is defined as the flow of energy from hotter to cooler objects.
Molecules move or vibrate inside any object. When we heat the object, the object's temperature will increase due to the increase in the movement of the molecules.
Heat is an energy that increases the temperature of the object by inducing faster molecular movement.

When the temperature of the object is different from its environment, then heat takes place.
Heat is not a matter and doesn't occupy space. Like light, sound and electricity, heat has no weight and it is a form of energy. Simply, heat is the kinetic energy of the particles inside the object.
  
Heat is represented through the SI Unit "Joule". It is also measured as "Calorie".
Reference:
https://pixabay.com/fr/photos/la-cuisson-sur-feu-village-de-cuisson-5175531/
https://commons.wikimedia.org/wiki/File:A_Local_Taylor_ironing_cloth.jpg
https://pxhere.com/en/photo/1020207