Checked in online and received a boarding pass but am on a waiting list.

Hello everybody. I have a flight booked and paid for with Swiss from Johannesburg to Zurich then Zurich to London. Checked in online and received a boarding pass but it said that I am on a "waiting list" so can not select a seat and that I have to get my seat at the gate. Phoned support twice and was told all is ok. What exactly does waiting list mean?

EDIT: Also what are my rights in this regard if I dont get onto the flight. As far as I know they can rebook you for free but if all remaining flights are full what happens next?