Priority date refers to the earliest filing date in a family of patent applications.  

• If the earliest-filed patent application for a particular invention was a provisional application, then the filing date of the provisional is your priority date.
• If the first filing was a non-provisional application, then the priority date is the non-provisional filing date.
• If the earliest-filed application was an international (PCT) or foreign-filed application, then the filing date of the non-US application is the priority date.

For example,

If:

• Provisional application filed on Jan. 20, 2016 disclosing features A and B of an invention;
• Non-provisional filed on Jan. 20, 2017 claiming priority to the 1/20/16 provisional and adding new feature C (i.e., disclosing A, B & C); and
• Continuation-in-part (CIP) application filed on Aug. 20, 2017 adding new feature D (i.e., disclosing A, B, C & D).

Then, the priority date for each feature would be:

• Features A & B: 1/20/16 (filing date of provisional);
• Feature C: 1/20/17 (filing date of non-provisional); and
• Feature D: 8/20/17 (filing date of CIP).

The priority date determines whether another patent filing or publicly available document qualifies as prior art against one's patent application.

(Source: http://www.patenttrademarkblog.com/priority-date/ accessed 9/25/2019)