Answer:
Thermal energy refers to all the types of energies related to the concept of "heat". For example, a fire has some energy in the form of thermal energy.
In thermodynamics, the thermal energy is directly related to the internal energy (U) of the system.
Another way of seeing the thermal energy is as electromagnetic energy, for example, the heat that you feel when the sunlight hits you is caused by the radiation of electromagnetic waves (visible light, infrared, ultraviolet, etc) the sun, so the thermal energy is directly related to the electromagnetic energy.