In order to keep track of which frame of video/footage is which, SMPTE standard timecode is used. Timecode shows up in a format of hours:minutes:seconds:frames, like 01:20:03:29. Timecode has [many forms](http://en.wikipedia.org/wiki/SMPTE_time_code), but fortunately the only two you’ll run into on a stage are LTC analog time code, and digital timecode embedded into the vertical ancilliary section (VANC) of the HD-SDI video signal.
LTC is described here (http://en.wikipedia.org/wiki/Linear_timecode), but all you need to know is how to embed it into a digital audio signal, and connect it to Previzion. This is typically done with an embedder, like the AJA HD10AMA. There are some details on the connection at http://lightcrafttech.com/support/documentation/takes-interface/timecode-source/.
The other (and easier) way of getting timecode is when it is already embedded into the HDSDI signal in the VANC area. Recorders like the SRW-1 already do this, so if you are working on a shoot that uses SRW-1 recorders, you just plug into their HDSDI Monitor output, and the timecode will already be there.
Previzion is set up to automatically recognize both VANC and LTC incoming timecode. If LTC is present, it overrides VANC.