The bosses in Dragon Ball Z: Kakarot have plenty of health and if you get involved in a drawn-out battle with one you could end up with very little health. Make sure before you confront them you delve into the menus and equip health restoratives to your bar so you can use them in the upcoming battle. If you don't have any health restoratition items you may want to consider exploring the local settlements for an item vendor where you will be able to pick some up.