A dial-in bridge is one where everyone dials in, rather than the bridge dialling out to sites at the start.
ALL sites, including the meeting host, must dial in. Think of it as a hub or virtual meeting room that everyone goes to; you all dial in and see each other.
It is more flexible than one we dial out to: you can dial in whenever you want (after the start time), and it does not matter to us where you dial in from, if you change your room, or don’t join at all.