The agenda for informal telechats is made (informally) by us, with things we decide we need to talk about. The agenda lives on the IESG wiki, here: InformalAgenda
We'll often use the informal telechats to have discussions about documents that are stuck (a chance to discuss DISCUSS positions in detail), or about other things that have come up. The informal telechats usually don't take the full 2 hours, but we block the full time for them just in case.
The Secretariat produce narrative minutes of informal telechats. These are posted on the private wiki and not made available to the public. That's handy for reviewing what we discussed, or for ADs who couldn't be on the call.
Feel free to add things to the agenda. If you see that it still has an old agenda, also feel free to clear it out and change the date appropriately, to set up for the next one.