Target creature gets +2/+2 until end of turn. If it's an enchanted creature or enchantment creature, it also gains hexproof and indestructible until end of turn. (It can't be the target of spells or abilities your opponents control. Damage and effects that say "destroy" don't destroy it.)
Whether the target creature is enchanted or an enchantment is checked only while Karametra’s Blessing is resolving. If it becomes enchanted or stops being enchanted later (or, in some unusual cases, becomes or ceases to be an enchantment later) it won’t gain or lose hexproof and indestructible.