You beseech an otherworldly entity for aid. The being must be known to you (ex: a god, primordial, demon prince). That entity sends a celestial, elemental or fiend to aid you, appearing within range. If you know a specific creature's name, you can request that creature.
When the creature appears, it is under no compulsion to behave in any particular way. You can ask the creature to perform a service in exchange for payment, but it isn't obliged to do so. You must be able to communicate with the creature to bargain for its services.
After the task is completed or agreed length of servitude expires, the creature returns to its home plane after reporting back to you. If you are unable to agree on a price for the creature's service, the creature immediately returns to its home plane. See PHB for more details.
Invito Amica Planorum