Table used to add restrictions for learning spells from trainer. Player can't learn spell spell_id till he learns req_spell, when he loses req_spell spell_id will be lost too. Table is used also for profession specialisations as specialisations require profession of certain rank to learn.
SpellID from Spell.dbc, which require knowing req_spell to learn from trainer.
SpellID from Spell.dbc, which is required to be known before spell_id can be learned from trainer.