1) "Open" means there is no security, anybody can call the number you have been allocated for the conference call.
2) For "Caller ID or PIN" security, if you specify a phone number for a participant and they call into the conference from that number, they will not need to enter the PIN, in all other circumstances participants will be prompted to enter the PIN.
3) For "PIN" security all participants must enter the PIN number when prompted.
4) For "Caller ID" security, only calls from the phone numbers that you supply for the participants will be allowed into the conference. You must supply phone numbers for all participants when using "Caller ID" security.