https://www.gamespot.com/articles/psa-red-dead-redemption-2-treasure-map-pre-order-b/1100-6461148/

One of the perks of pre-ordering Red Dead Redemption 2 is that you get a digital treasure map that shows the locations of "secret buried treasure" throughout the world. If you're interested in this perk, you have to pre-order soon. Rockstar is only giving out the treasure map to those who pre-order by August 15, which is coming up very, very soon.

The treasure map is only available for digital pre-orders from the Xbox Store and PlayStation Store. There are no other details available about what kind of treasure you'll find, but Rockstar says you'll find "hidden riches" in the single-player world; the map is for single-player, not multiplayer.