It will call a number of small creatures. What species it summons is apparently random; some may have high skills but some have relatively poor skills. It seems that the only standard for the summoned creatures is to have wings. Using it alone does NOT increase or decrease your relationship with a creature you are allying or killing, but it helps you fight or befriend them more easily, if the creatures that join you have high combat or social skills.
It is similar to the Space stage consequence ability called Summon Mini-U, but Summon Flock gives you several allies to fight alongside rather than one.