A shelter is a rest area to respawn and regenerate your health and mood. If you carry a map in your rucksack you can see where the next shelter is located. There is also trail magic that will not be marked on the map like the bus on the image below.

Note: If you "really" die "grave" in singleplayer you will see your score and have to respawn from the start otherwise you spawn from the newly discovered shelter.

Magic Bus
Magic Unfinished Hut
Tarp Shelter