Room Name : -

Room name is used to identity a unique web meeting room.
Currently we use the Dimdim id of the user as his/her room name. Room name can’t be changed by the user and is unique for each user. 

An attendee needs to know only the Room Name to join a public Dimdim meeting.  The host can send emails to invited guests using the following URL where "roomname" is simply the user's DimdimID

http://webmeeting.dimdim.com:80/portal/JoinForm.action?confKey=roomname

Anytime the host is in a live meeting, anyone can join by via this URL or going to Dimdim.com, clicking the "Join meeting" button and entering the meeting room name.
 
Meeting key : -  

Dimdim meetings can be made more secure by requiring a meeting key or password.The meeting key can be different for each meeting. Attendee needs to provide both the room name and the meeting key to join a private meeting.  The meeting key is given by the Host when he start the meeting. It is recommended you create a different meeting key for each private meeting.

Host key : -  

Host key is specified by the host of the meeting before starting the meeting as a way to ensure security.
This is typically used as an additional security key which can be used by the host to rejoin the same meeting. For instance, if the host loses his/her web connection or their browser crashes for some reason during an active meeting, simply click to restart the meeting and enter your host key. 

Even if you are hosting the meeting you’ll be able to rejoin without fear of the meeting closing on all the other attendees.  Host key should be different for each meeting.

To set the host key and the meeting key , please refer this workflow -

1) Sign in to your Dimdim account.
2) Click on "Host Meeting" button.
3) Click on "General" tab and enter the meeting key and host key.