[Libav-user] what are pts and dts ? what does this code bit do in ffmpeg.c ?

[Aditya]

what are pts and dts values. why are they important while transcoding .
what is the purpose of the below code bit 
http://pastebin.com/XwNechy2
in ffmpeg.c 
http://cekirdek.pardus.org.tr/~ismail/ffmpeg-docs/ffmpeg_8c-source.html