This setting controls the build priority for the associated project as considered by the NCrunch processing queue.
A higher priority will cause NCrunch to try to build the project earlier, compared with other tasks in the queue.
When the project needs to be built as consequence of changes to another project, the effective priority value will be automatically divided by 4.
This setting is useful when working with projects that make use of the copy referenced assemblies to workspace setting but do not have a high importance attached to them in the context of test execution. Installer projects often fit this description.
The mechanics of the NCrunch processing queue are such that any tasks will receive an automatic priority boost if they are the dependency of other tasks with a higher priority. This is done to ensure that the engine always takes the quickest route to executing high priority tests. It's worth considering this when trying to micro-manage build priority, as the engine will very often override this configuration setting in an effort to give the most relevant feedback quickly.
Unless you have a very specific performance bottleneck around the building of specific projects, it is usually wiser to leave this setting at its default.