Thermal Protective Performance is essentially a measure of how long it will take to obtain a 2rd degree burn when exposed to a heat source. A 2nd degree burn is a partial thickness burn that involves the epidermis and the dermis (the first few layers of your skin). The test is done by placing the layered material to be tested over a radiant heat source and an open flame and recording the time it takes to transfer enough energy to cause a second degree burn. This is done on a 4 inch piece of representative material and the time in seconds it takes to reach the burn threshold is then multiplied by the heat flux to get the final value.
This is an example of the device used for testing (Thermetrics, https://thermetrics.com/products/protective/tpp-htp-test-device/)
This is a useful number for the end user as it gives an objective number for protection. The NFPA mandates that structural firefighting gear have a minimum TPP of 35, although this is typically higher. In motorsport, the SFI has a useful system that allows for easy identification of the TPP of a garment. The suits that most drivers you see on TV wearing at the level 5 suits that are rated for a TPP of 19-which can be thought to give them a minimum of 10 seconds to escape a fire hazard in the car. Drag racers wear higher rated suits due to the type of fuel they run in their cars. You can usually find them in SFI 3.4A/15 or SFI 3.4A/20 suits.
(SFI, https://sfifoundation.com/article-1/)
In Summary, the TPP is a measure of insulation that can be used to make decisions about protection levels of garments given a specific known hazard. The rating and certification systems can be misleading at times as the certifications allow for different TPP levels in different portions of the garment. This underscores the importance of knowing your gear and what it is capable of prior to entering a hazardous situation.
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