The founder of a room can lock the modes of the room by using the mlock command. When a room is registered a default mlock of +rtn-k is set. See the room modes list or use /hs cmodes to see what each mode does. The modes that are mlocked on, with the + sign, will be set by services after the room empties when the first person rejoins. Services will also prevent anyone, regardless of their status in the room, from turning those modes off. If the founder wishes to change them he/she must first change the mlock. Services will remove any mode set for the room that is mlocked off, which is symbolized with the - sign. Some modes can only be set by network operators (such as M) and some modes cannot be locked on (k and l). If you try to lock a mode that cannot be mlocked the mlock will be set without those modes in it.
Example:
/msg chanserv set #conferenceroom mlock +stn-kl
Mlocks are useful for defining what kind of room you are running. If you're not sure whether some modes might be needed for management, do not mlock them positively or negatively. If you know that you never want your room to be secret or private, then you can mlock it to include -sp. Similarly, you can mlock modes you never want off, such as n (no outside messages) which almost all rooms will want to keep. Mlocks allow you to set up your room once and then not have to worry about it, since services will maintain it the way you want it.