Todash Space is the void that exists between worlds. It is entirely dark and is filled with monsters.
Travelers that go todash can end up in todash space if they fail to make it completely back to their reality. There is also a Door underneath Castle Discordia that leads to todash space. The Old Ones who made this door considered it to be a mistake, but the Crimson King took full advantage of it. He threw his enemies through this door, to wander the space, deaf and blind, until one of the todash monsters ate them.
One of the todash monsters escaped todash space to roam the tunnel between Fedic and Castle Discordia. This tunnel demon nearly killed Roland Deschain and Susannah Dean, but they were able to fend it off by using Sterno-fueled torches.
It is possible that the Macroverse inhabited by It and Maturin in Stephen King's It is somehow related to todash space. The "empty channels" of the Otherspace teleportation device in The Tommyknockers may be related as well.
In From a Buick 8, the Buick acts as a portal to another dimension, possibly the Todash Space, and allows various creatures and monsters to come through. However, the true nature of the Buick is never explained.