Before parties embark on missions or quests, they often check to see if every member of the party is ready to go. Instead of saying "ready," players often type the number "1" to indicate their readiness. This originated from party leaders asking the members of the party to "type 1 when ready."
In some parties, members may be asked to indicate readiness by typing the number corresponding to their position in the Party List in order to make it easier to keep track of who has not responded.