The Dark Tower Wiki

Todash is a place of being that exists between universes, and that a person travels through to get from one universe to another. The todash state is usually accompanied by the "kammen" or chimes, which produce unpleasant or pleasant sound similar in effect to a thinny.

Alain Johns went into todash twice when entering Maerlyn's Grapefruit, trying to save Roland's soul which was trapped inside. 

There are several ways that a person can "go todash", which can be used as a method of fast-travel if one is skilled and careful enough.

Going todash is very dangerous. It is possible to get stuck between worlds in the "todash darkness," from which there is no return.