What is S/PDIF Connection

My jitter filter has digital RCA and S/PDIF outputs. What is this S/PDIF, how used, any gains Thanks
S/PDIF is the unprocessed digital output, and has usually two interfaces: RCA or Coaxial (cable) output or Toslink (optical) outputs. Some CD players, and all CD transports, have one or both of these outputs. These outputs are usually connected to the input of an outboard D/A converter. Some digital processing preamps can also use this as an input, as they have their own internal D/A converter, such as the SONY 2000ESD DSP preamp. Your "jitter filter" takes your CD transport's S/PDIF output ALREADY as an input, one questions WHY are you re-clocking the CD-player's output twice, first with this "filter" and then again in the D/A converter? There is no point in doing this, why not go directly from the CD transport to the D/A converter, it's all just S/PDIF anyway. And if you are using a CD player, which has it's own D/A converter, why use and outboard converter? If you have purchased a "jitter filter" then you, sir, have been swindled!
Regarding the jitter filter - Fotopres is off base on this one. But first let me answer your question: S/PDIF stands for Sony/Philips Digital Interface Format. As Fotopres said, this is the digital output from your transport that goes into your DAC. He is also correct in stating that this ouput is clocked with a timing signal when it comes out of your transport. However, these timing signals can have errors (ie they are not evenly spaced). These timing errors are generically known as jitter and supposedly result in the analog waveform being distorted after it is decoded in the DAC. Jitter has many supposed sources like vibration, noisy systems and interconnects and poorly engineered circuits. A Jitter filter decomposes the digital signal into its clock and music components, regenerates the clock signal (hopefully a more accurate one, but some jitter filters have been shown to actually increase jitter), and then recombines the clock and music signals and sends it to the DAC. I recently added a Monarchy DIP between my transport and DAC and noticed a big difference right away, not a subtle one. The music became fuller, the tones rounder - like spheres instead of blocks. I've spoken with people that have put 2 of the DIPs in series and they claim this improves the sound still further. After the improvement I got from my first one, I'm considering adding a second one. Again, let your ears tell you whether you hear a difference. By the way, you can order the DIP from AudioAdvisor.com for $199 plus shipping and they have a 30 day money back guarantee - If you don't hear an improvement, send it back. I'll bet you won't. Now that I've said all of that, I suspect what you really meant was that your filter has RCA and AES/EBU (also known as balanced) outputs - the RCA output IS an S/PDIF output. If that is what you really then ask a question here about balanced outputs or see an excellent writeup in Robert Harley's book, "The Complete Guide to High End Audio".