Soulbound is enchantment that keeps the enchanted item in your inventory after death.
Can be obtained through enchants of level 1 only on a spellbook. It does not work if the mod Mine Factory Reloaded is installed. You will need to install the mod Enchanting-Plus and use the Auto-Enchanter to get the enchant Soulbound.
When you craft some items, like the Earth Chestplate, it automatically comes with Soulbound 1.