Priority mail is a type of service offered by United States Postal Service (USPS). Packages shipped via Priority Mail usually arrive to their destinations within 2 to 3 business days. Priority mail offers free shipping materials and flat-rate shipping as well.
Priority mail is when you pay extra money for what you want to mail. The office then guarentees that the mail you are sending gets there on a certain day. I recently mailed something priority mail on a Wednesday. They told me it would arrive on Friday or Saturday. it probably would have gotten there on those days anyways.