The DCD line is a 'Modem Control' line. Not usually used when connecting two computers or other 'Terminal' equipment.
When using a Modem - your modem dials a number and negotiates a connection to a remote modem and this property will become true once the connection is established. Once this property becomes True it means that the modems have finished negotiating and you have a through connection to the remote computer and any data you send will be sent, via the modems, to the remote terminal.
If this property unexpectedly goes False during a modem connection then it means that your connection was lost.