Priority Date

The priority date is the filing date of the very first patent application for a specific invention.

Within 12 months of that first filing, a subsequent patent application for the same invention can be filed claiming this "priority right".

According to the Paris Convention anyone who files a patent application, can within 12 months (the priority year), apply in another country and claim priority from the first application. 

When it comes to novelty, priority means that your later application can be seen as if it was filed the same time as your first application. The priority date is the filing of your first application.