The Heat Resistance (HR) attribute represents how well your ship is able to withstand the immense heat that is present near the Sun. Currently it's only use is to be able to visit Mercury. In order to do that you need to get your Heat Resistance to or above the score of 2,000. You can increase your Heat Resistance through equipping Utility, Armor, Sensor, Weapon, and Shield Mods with the Heat Resistance attribute, which can most easily be found on Jupiter.
No ships come with a base Heat Resistance, with the exception of the JayCo Tanker at 2,000 Heat Resistance.