When two objects with different temperatures, there is a heat transfer process between them. The heat will be transferred from a higher temperature object to a lower temperature object until the temperature between them is equal to the state of thermal equilibrium. This process of heat transfer is called thermal process.
When an object is heated, the object absorbs heat and heats up until the heating energy is equal to the losing heat at a high temperature.
For example, when a baking furnace is heated, air and parts of the furnace body will quickly absorb heat to increase the temperature. Because the furnace body is higher than the surrounding air, there is continuous heat leakage and transfer to the air. The higher the temperature of the furnace body, the more heat is lost into the air. When it reaches a certain temperature, the heat loss of the furnace body is equal to the heat emitted by the heating part. The temperature of the furnace body will stop to increase.
Therefore, all parts of the furnace have reached thermal equilibrium. In thermal equilibrium, there are still thermal processes, but the temperature of each part is basically stable.