Temperature is an environmental game mechanic that, like Radiation, affects the player. In most circumstances, extreme temperatures are harmful to the player. However, unlike Radiation, the player can also harness heat sources for Energy applications.
High Temperatures are typically generated by underwater volcanic activity. Such sources of heat include Lava Geysers, which can be found in a few biomes, and Thermal Vents, which can be found in some of the deeper biomes. Certain biomes are also quite hot, such as the Inactive Lava Zone and Lava Lakes. Direct contact with lava will instantly kill the player.
The player starts taking damage from Temperatures above 50°C. The UI will indicate that the player is too hot by showing glowing orange lines on the edges of the screen. However, the player's thermal resistance increases to 70°C if they're wearing the Reinforced Dive Suit. Note that water near lava in the lava zones is significantly hotter than elsewhere within the biome, and is above the 70°C that the suit can protect against.
To survive any more extreme Temperature will require the use of a vehicle, particularly the Prawn Suit. The Prawn Suit is especially useful for collecting resources near heat sources that are too dangerous for a player on their own. Though again, direct contact with lava will damage the Prawn Suit.
Heat can be used to generate energy for Seabases through the Thermal Plant; because these are external generators that have to be placed at specific locations to work (any source above 25°C), the Seabase needs to be close, or otherwise connected via Power Transmitters.
Prawn Suits can generate energy from heat using the Thermal Reactor upgrade (any source above 30°C). The Cyclops is also capable of this, if the vehicle has the Cyclops Thermal Reactor Module installed (any source above 35°C at the engines).