The mid is the number you see quoted by news sites and search engines, and the one money-transfer services mean when they advertise the real exchange rate. It is a reference point, not an executable price.
Marking positions to the mid flatters your account, because closing always happens on the worse side of it. Many platforms mark unrealised profit at the mid and only show the true cost when you exit, so a position can look green while a round trip would still lose money.
Example: EUR/USD is 1.08400 / 1.08424, giving a mid of 1.08412. A trader who bought at the ask and sells at the bid is down 2.4 pipettes, roughly $24 on a standard-lot, even if the mid never moved.
Related: two-way-price, bid-ask-spread