Thermal means how much heat the motor can handle. This is often associated with starts/stops the motor is designed for, the ambient temp, the running amps, etc.
The thermal capacity of the motor is often simulated with protective relays to know how long the motor can run, based on starts/stops and running amps. For example, it starts twice an hour and is running slightly above nameplate amps. After two hours, the motor may only be allowed to start once an hour because it is at its thermal limit. Thats a crude example but hope it helps.