A Thermal Pad is a pre-formed square found on the heatsink as part of the Cooling Bridge. Its main purpose is to conduct heat away from the Raspberry Pi.
All new purchases of a pi-top  come supplied with the thin Thermal Pad affixed to the heatsink on the Cooling Bridge to work with the Raspberry Pi 3B+.
To check if you have the correct Thermal Pad, simply turn off your pi-top and remove the Cooling Bridge. If you are using older models of the Raspberry Pi you would require the thicker Thermal Pad affixed to the heatsink.
If you have the wrong Thermal Pad, you may experience resistance when inserting the Cooling Bridge, Screen issues or you may see 3 flashing red LED lights on the Hub indicating the Cooling Bridge has not been inserted fully.