A priority date is achieved when you are the first to file a specific innovation within a country. The filing date is considered the “priority date”. Once filed you are entitled to claim priority for a period of twelve months. Therefore, when you apply for protection in other member countries during those twelve months, the filing date of your first application is considered to have “priority” over other applications filed after that date.
In such a case, you still succeed in being the first-to-file in other member countries, even if there are other applications filed before the filing date of your application in those countries as you have the “priority date”.