No, it's not a poison ivy fest--it's the meeting of the referee, coaches, and team reps 10 to 15 minutes before the meet begins to substitute for missing swimmers.
The basic rule is a team may only substitute for three swimmers after the entries are exchanged around noon on Friday. If four swimmers are missing, the fourth's lanes just go empty. This is why it is so important to enter only kids who intend to come to the meet. If Johnny is away at camp and you enter him anyway because you don't realize he's gone, you've got an immediate hole to fill. If Joan goes to the doctor Friday afternoon with a terrible case of swimmer's ear, Susie falls and breaks her arm Friday night, and Billy starts throwing up in the wee hours of Saturday morning, you've got big problems. You can "scratch" and substitute for three of those swimmers, but not all four.
You can only substitute for three swimmers in the line up, but you can use multiple swimmers to fill the empty slots. For example, if Susie is entered in the individual medley, freestyle, backstroke, and fly, you can put Mary in Susie's IM and free lane, Cathy in Susie's backstroke lane, and Heather in Susie's fly. If Mary, Cathy, or Heather are already entered in the meet, they have to stay in the events they are in--so if Cathy's entered in free and fly, she has to stay in those events and just add the back. If Cathy is already in free, breast, and fly, she can't go into Susie's lane in back, because she can't swim all four strokes--if you don't have another swimmer available, Susie's backstroke lane goes empty. Mary, Cathy, and Heather don't have to be in the meet already, but they do need to be on the team roster.