Heat is a form of energy that can be transferred between objects or systems. When we say that something is "hot," we are describing its temperature, which is a measure of the average kinetic energy of the particles within that object.
While an object may not contain heat itself, it can gain heat from its surroundings through the process of heat transfer. Heat transfer occurs when there is a temperature difference between two objects or systems. Heat will naturally flow from the object with a higher temperature to the one with a lower temperature until thermal equilibrium is reached.
For example, if you place a cold object near a hot object, heat will transfer from the hot object to the cold one until both objects reach the same temperature. This is because the higher-temperature object has more kinetic energy in its particles, and this energy is transferred to the lower-temperature object, increasing its average kinetic energy and thus raising its temperature.
So, something becomes hot when it gains heat from its surroundings or from another object with a higher temperature. The heat transfer process raises the average kinetic energy of its particles, resulting in an increase in temperature.