Summons a creature to help you.
In the past players used to be able to summon an unlimited number of creatures including stronger creatures than today like Dragons which can no longer be summoned. When the player logged off or disconnected the summons would turn into wild monsters that behaved the same as a normal creature.
In the early days of Tibia, when only Antica existed, even Paladins were able to summon creatures. When the ability was removed, the old paladins did not loose the spells learned from it. The remaining paladins that could summon creatures lost their spell on Update 10.55 when vocation requirement was also applied while casting spells, not just while buying them.