The prioritization defines the order, how filter rules are applied. If you set up too many filter rules, it may happen, that some of them are not applied.
You can set the order in which the rules are applied using different priorities. You can create any number of filter rules. In some cases, filter rules affect or contradict one another. In cases like this, the rule that is applied first - the higher priority rule - "wins".
Example: Filter rule a) wants to delete an email, but filter rule b) wants to move the email to a folder. Rule a) has a higher priority.
In this case, the higher priority rule a) is performed first, and the email is deleted. Deleted email cannot be moved, so rule b) is not performed. The "Your Filter Rules" list is sorted by priority. As a result, it indicates exactly the order in which the rules are performed.