Hello and welcome to Warmides boards,
As the description says about shelter :
The Shelter can protect your Workers and some Materials from an emergency,hidden by the enemy's eyes.
As the Shelter's level increases , the storage capacity for Material and Worker increases.
This means that every time you camp is under attack, your resources and workers are protected inside the shelter (thats why it is "hidden" there).
So if your shelter is at Level 1 it will protect 300 resources from each resource ( thats 300 oil,300 coal,300 iron and so on) and at the same time, it protects 10 Workers who will survive the attack.
I hope i helped you understand its usage. If you still have question feel free to ask.
Have a nice day,
Midas
Warmides board support team.