Hello! Allow me to help!
Your question: What is thermal energy?
My answer: Internal energy present in a system in a state of thermodynamic equilibrium by virtue of its temperature. Thermal energy cannot be converted to useful work as easily as the energy of systems that are not in states of thermodynamic equilibrium.
Want to know why my answer is right, or simpler terms of what I stated above? Allow me to explain: Heat energy, also called thermal energy, is the energy an object has because of the movement of its molecules, and heat can be transferred from one object to another object. Heat energy on Earth comes from the sun.
Hope this helps! UwU
-Maxwell