Priority is one of the factors that influences the dynamic scheduling of projects and tasks. Other factors such as task dependency and deferral dates do take precedence, but priority is the default order once these have been dealt with. Note that all active projects are scheduled before planned projects.
If a number of active projects are due to be scheduled, and they are all set at the same priority, then the scheduling order will be chronological based on the creation date of the project. Even if a project has fixed start and end times (ie. not dynamically scheduled), it can still be helpful to set an appropriate priority, as this helps to highlight projects in lists.
An arbitrary number of priorities can be defined, and be called whatever you wish. The higher the number, the lower the priority. A style can be defined for each priority in order to customise it's presentation.