Dealer invoice is the price on the invoice that the dealer pays Ford for the vehicle. These prices are published everywhere now - Edmunds, Kbb, etc. They are real but the online versions don't include certain things like dealer association advertising fees, fuel, etc. so the actual dealer invoice will be around $400 higher than what you see online.
The kicker is that Ford and most other mfrs give their dealers back some of that dealer invoice cost - it's called holdback. On Ford it's 3% - others are either 2% or 3% and some of the imports may not have it at all. So even if a dealer sold the vehicle at invoice they still make 3% (I think it's 3% of MSRP but it might be 3% of invoice). But you also have to remember that most dealers floor plan their vehicles - they basically borrow the money to pay for the vehicle and pay interest on it each month until it sells. That will eat into the holdback as will any other real expenses that the dealership incurs.
Except for rare occasions when the dealer wants to move a specific car (maybe it's been on the lot for 6 months) you can't buy a car below dealer invoice (again there are exceptions but they are rare. Won't happen on a popular vehicle). If it's advertised that way then one of 3 things is happening:
1 - there is a factory rebate which the dealer is keeping. If the rebate is $1K and the dealer sells it to you for $500 below invoice but keeps the rebate then they really sold it for $500 over invoice.
2 - there is unadvertised factory to dealer cash (sometimes called marketing support) that the dealer is using just like a rebate. Edmunds usually lists these under incentives and if you're buying on A/X or Z plan you normally get to keep that cash yourself.
3 - the dealer is making it up with an unusually large "documentation" fee - otherwise known as additional dealer profit. This is normally $300-$400. Or they're forcing you to buy add-ons (paint protectant, glass etching, etc.) at inflated prices.
Hope that helps.