CDCSS or Digital Code Squelch (DCS ) is a further development of the continuous tone-coded squelch system or CTCSS that uses a slow-speed, binary data stream passed as sub-audible data along with the transmission. Motorola calls this Digital Private Line or DPL.
DCS is a two way, 3-digit, digitally encoded code. Think of it as a two way password. You need the password to transmit and to receive a signal.