Soulbound is enchantment that keeps enchanted item in your inventory after death.
Can be gotten 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 Chest plate, it automatically comes with soulbound 1.