From FreeSpace Wiki
When (Conditional operator) Performs specified actions when a condition becomes true Takes 2 or more arguments... 1: Boolean expression that must be true for actions to take place. Rest: Actions to take when boolean expression becomes true.
This is the default conditional operator for an event, used for events that must turn true or false only once, or for those repeated events that don't make the event turn true infinitely. For that, see every-time.