When you start a game, you can choose between 4 different game modes: survival, free, extreme and creative.
In survival mode, you have to find materials and components to craft items, and manage your oxygen level, hunger and thirst.
In free mode, you also have to craft items, but you don't have to manage your hunger and thirst. However, you do need to manage your oxygen level.
In extreme mode, it's the same as survival mode, but you only have one life!
In creative mode, you have no more constraints: you don't have to manage your hunger, thirst, oxygen levels or pressure, and you can't die. Materials are not required for crafting.