They're consuming resources but not doing anything beneficial to the vault.

Dwellers who are on Coffee Break and considered idle. You can also sort them according to level, SPECIAL stat, status, and job. In this screen, monitor your dwellers and explorers so you can recall them if they run low in Stimpaks or see if there are dwellers on 'Coffee Break'. Tapping the gear icon on the top left corner of the screen will display the dweller list. Keep track of your dwellers and explorers However, it's still ideal to have a dedicated number of dwellers working in their respective production rooms with high individual stats and happiness level to make the most out of your production rooms. You can also temporarily move workers with fairly passable SPECIAL stats from one production room to the room that's suffering the shortage. You can solve this problem by adding/merging production rooms. If you can see one particular meter draining rapidly, check if you don't enough enough facilities or if your current production rooms are not producing enough resource compared to your Vault's needs. Having several production rooms means nothing if you don't have enough workers to fill them. That said, build your rooms according to your Vault's current population. It will also require more dwellers working in them for them to run efficiently.

Merging up to three rooms gives you the most out of your production, storage, and a bit cheaper upgrade cost. However, in the long run, you'll need to destroy ('sell') some of your rooms so you can merge them. It's easy to place the rooms haphazardly in the game, especially during the early parts of it.

Fallout shelter special guide