Pale Marks are a form of currency used by the Pale Hand.
In-game
Pale Marks were introduced to Diablo IV in Vessel of Hatred, and are earned exclusively through the use of Pale Hand mercenaries in the field—100 Pale Marks are given at various rapport levels as rewards. When a mercenary is maxed out at Level 10 Rapport, players can 're-level' to gain 100 Pale Marks again.
Pale Marks are used in the bartering system to purchase items from Fayirah.