Dragonmarks are variant races (found in Eberron: Rising from the Last War), so they are mutually exclusive with Variant Human(For humans, the Dragonmark replaces the Ability Score Increase feature, which Variant human also replaces). Dragonmarks grant(among other features), the ability to cast two spells through the Dragonmark (though you do not learn those spells) and a list of 1st through 5th level spells are added to the class spell list for all your classes.