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, is not affected by nor . As another example, cannot gain stealth (since it's an ability) or power (since it's a characteristic).
Note that 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.