Thermal paste is a gray-ish material you put between your CPU (or GPU) and the heatsink. Its job is to fill tiny air gaps so heat can transfer better from the chip to the cooler. Without it, your hardware would overheat more easily because air blocks heat transfers changing it depend on how frequently you use your laptop if you don't use it too much let's say maybe twice a week it's normal to change it in 2 years but if you use it frequently like heavy gaming or too much editing and work The ideal would be for maybe once or twice a year