A roof valley is a term used to describe the part of the roof where two slopes meet. Water collects in this area. When the valley is in good condition, water flows down the valley so that it does not accumulate on the roof.
In most cases, it's best to approach DIY plastering with a two-coat approach, which allows you to lay the perfect foundation for your final coating. The best thing to do is to apply a simple, thin coat to start with, and this will go a long way towards giving you a smooth, flat finish for the finishing coat that you'll be applying later on, see this site for more.