When something is indescribable, you cannot put it into words. The feeling or the scene is so strong, so strange, or so big that ordinary language is not enough. People use it for very good things and very bad things: an indescribable joy, an indescribable smell, an indescribable sadness. It is an adjective, a little strong and a little bookish; you will meet it in stories and reviews more than in a short text to a friend. Though actually, in everyday talk people also use it loosely, just as a heavy “really, really” — “The view was indescribable” often means “it was amazing,” not that the speaker truly had no words left. It does not mean “has no name” or “cannot be talked about because it is secret.” It means the thing itself is too much for a neat description.