When thermal energy is added, the motion of particles increases.
This can be thought of when thinking of a gas. We know that a gas includes particles that move fast and are spaced far apart from each other. On the opposite side of the spectrum, we know that a solid includes particles that are closely packed together with limited movement.
Using those examples, we know that a solid usually occurs at colder temperatures (with low thermal energy), and gases occur at warmer temperatures (with high thermal energy).
Therefore, when thermal energy is added, particle motion increases.