Essentially, it is forcing foreign financial institutions to play snitch for the US Gov.
The burden is on the foreign account holder to prove they are not a US Person and if they are, to file the documentation they technically should have filed with the IRS.
The USG will now connect US passport/Tax Payer ID to the National ID number of the US Person in whatever country. The USG will be able to access info on the financial (and other) activities of that US Person at will (or at least once a year) in that country.
US Person
There are many persons who were born of US parents and never lived in the States or only temporarily as a small child. Many probably do not know they technically should have been filing taxes. I recently read that there is another "generous" IRS amnesty program that will finally pull these people into the USGs radar and extortion ring.
For the rest of us grown US Citizen expats and Green Card Holders, the same info has probably already been forwarded by the foreign financial institutions where we have accounts (if you have not lost your foreign account due to being a "US Person", then they are complying with FATCA).