It is an abbreviation for station message-detail recording.
Regardless of their size, most telephone systems output Call Detail Records (CDR). Generally, these are created at the end of a call. But on some phone systems, the data is available during the call. This data is output from the phone system by a serial link known as the Station Message Detail Recording port (SMDR).
Some of the details included in call records are: Time, Date, Call Duration, Number dialed, Caller ID information, Extension, Line/trunk location, Cost, Call completion status.