On this page of the guide to Void Bastards, you will learn how Heart Starters work. They protect your character from death and allow you to continue the game without playing as another random prisoner.
Unfortunately, Heart Starters are not available by default, and to change this, you need to purchase the Heart Starter project (see the picture). It becomes available only after unlocking its previous upgrades - Padded Jacket, Ammo Subsidy and Travel Subsidy. You also need Surgery 4 Dummies and Control Collar parts.
Check the STATUS tab - the game will confirm that you have received one Heart Starter (see the picture above).
After you buy the mentioned project, you will get 1 "free respawn". The Heart Starter will activate automatically after you die during the mission. You will return to the battle with a full health bar.
An important feature of the Heart Starter is that it can be used once. Because of that, you should treat it as a last resort item, not something you should rely on regularly. The good news is that you can improve the Heart Starter project to get more starters.