耐热 is a stative verb (s.v.) meaning 'to be heat-resistant' or 'to withstand high temperatures'. It describes materials, objects, or even biological traits that can endure heat without damage. You'll see it in product labels (耐热玻璃 'heat-resistant glass'), cooking contexts (耐热容器 'heatproof container'), and technical descriptions. It's not used for people's tolerance of hot weather — for that, use 怕热 or 耐高温 differently.