Sorry for being obtuse, but what is yeast water?
Yeast water is natural yeast cultured from fruit pieces left in some water. This is as opposed to sourdough, which is yeast cultured from milled grain left in some water. It is basically the first step in primitive wine-making, but instead of letting the yeast ferment all of the fruit sugar into alcohol, at which point the yeast would normally die, the actively reproducing yeast are introduced into flour and water and used to leaven dough.
Also note you don't get a sour flavor with yeast water but you do get a nice sweet flavor and open moist crumb. It's easy to do and there are many posts on this site with directions.
Thanks for the info!