Q:

What is S/P and P/S converter in communication?

I have a scheme that in it I have D/A which is digital to analog converter, and A/D but what is it P/S and S/P ?
And what is it used for?

A:

"Parallel to Serial" and "Serial to Parallel" respectively.
Serial communication offers several advantages, that are usually were (in the past, when frequencies were lower) paid by slower speed:

Less wires (first and obvious, and influencing the others);
Avoids the problem of sychronizing the signals, that in the parallel connection can be skewed;
Generally more robust.

As Kevin said, serial connections are getting faster and faster, and serial protocols like PCIe and SATA have taken the place of parallel BUSs. Mostly this is due to the fact that the limiting factor for parallel connections is the capability to synchronize the signals over the various lines, while serial protocols haven't this problem.

