Elite Dangerous introduces the concept of heat - something that was absent in previous games, but which is a real-world challenge for space travel as there is no way to conduct or convect heat in a vacuum meaning that special measures need to be taken to remove heat from your ship.
Every component of your ship adds to your heat as it uses power, and the hotter you are, the easier it is for other ships to spot you on their scanners. The current heat signature of your ship is shown adjacent to the power distribution meters in the cockpit, and the current heat generation is shown on the gauge to the left of the scanner.
High power consumption activities like super cruise or hyperspace jumps will add huge amounts of heat meaning that you are easily visible to other ships when you exit super cruise or a hyperspace jump; conventional propulsion produces significantly less heat.
If you cut your heat generation down to zero (by disabling sub-systems) your cockpit will ice up!
Your ship has automatic heat vents that will open and close to manage the heat - you can chose to force these vents to stay shut to avoid leaving a tell-tale heat signature (aka “silent running”), but the heat is still building up and will need to be released at some point.
If the heat is not released you will start to notice that your cockpit is smoking and sparking and your ship will start to take damage until you explode. The heat gauge to the left of the scanner will tell you how close you are to a thermal failure of your ship.
Upgrades can be added to your ship to help with heat management. The main one of these are heat sink modules that allow you to “jettison” heat from the ship via heated metal slugs; these have the benefit of ejecting heat, and also of acting as a decoy on other ships' (or heat-seeking weapons') scanners.