A card that 'can't be modified' cannot be disabled, silenced, immobilized, or transformed. It also cannot gain or lose any abilities, and its characteristics cannot be changed. For example, Bedrock is not affected by Atlantean Fate nor Abundance. As another example, Monks of Kobalsa cannot gain stealth (since it's an ability) or power (since it's a characteristic). Note that Monks of Kobalsa can have their cost reduced (or increased) during casting while they're still in hand, since abilities only apply while in the realm unless explicitly stated.