What is Iris messaging and how does it work in chapter 15

Annabeth decides they need to contact Chiron, so she leads them to a car wash, where they pay to use a spray nozzle so they can "Iris-message," or IM. The spray mist makes a rainbow and Annabeth offers up a gold Greek drachma to Iris, the goddess of rainbows. An image of Half-Blood Hill appears, and Luke is the first person they see. He says that Chiron had to go break up a fight, and that campers are starting to take sides in the Zeus-Poseidon standoff and things are getting ugly. Annabeth and Grover go to deal with a car that pulled into the car wash while Percy finishes the conversation with Luke. They finish talking quickly as the mist fades and Grover and Annabeth come back, and Percy never does get the chance to talk to Chiron.