Adventurer's Armchairs are a substitute for beds in the Erebus. If you need to ask why you can't just use a Bed, try placing one. No, it won't explode if you try to sleep in it, but it'll end badly nonetheless.
In order to use an Adventurer's Armchair to reset your Erebus spawn point, you must have Erebus Respawning turned on in the config file (which can be found in your .minecraft/config folder). As long as that is turned on, when you right-click on the armchair, your character will sit in it and reset your spawn.
Obviously there's no day/night cycle in the Erebus, so no time will actually pass, beyond how long you decide to remain in the armchair.
It is made using two Varnished Planks and four Wool of any colour.