To summon is to formally call for the presence of someone. If, as soccer team captain, you find that your team members are an hour late for the big game, you might need to summon them — and fast.
Summon can also mean “gather up,” as one might summon one’s courage" before that big game. Summon is also occasionally used in reference to the supernatural, as when one tries to call forth a spirit or ghost. If you think the team needs additional help on the field, you might try to summon some supernatural backup.