CTCSS is an acronym for Continuous Tone-Coded Squelch System and is a system that is used in shared two way radio systems. It is common for a single radio channel to be shared over a number of user groups for frequency use efficiency. CTCSS uses a special tone to identify each group of users, and this tone is used to prevent a group hearing radio chatter from another group sharing the channel.
Over on the SDR# Yahoo Groups forum, a CTCSS detector plugin for SDR# has been posted. The plugin can detect CTCSS tones and provide a squelch for the desired tone. The latest version of the plugin can be downloaded from this link.