A 12b-1 fee is an annual marketing and distribution fee charged by some mutual funds and ETFs. This fee is included as part of a funds expense ratio.
You can think about this as the marketing budget for the fund. This is how much the fund spends trying to get more people to invest.
For most (if not all) Vanguard funds this fee will be 0%. The limit for a 12b-1 fee is 1% for any fund.