PIDs are those numbers at the end of any internet list that determine whether your system is up to receiving a channel or bouquet after the normal parameters of frequency, polarity Symbol rate, and FEC.
If these are correct then more often than not you receiver will pick up the channel automatically, name it, asign an audio list for multiple languages and bring in the right number of radio channels with it.
Sometimes however the awkward channels will be broadcasting slightly outside parameters that are deemed DVB compliant. When this happens you can on most receivers go into an advanced menu to set these PIDs in Decimal or (less commonly) Hexadecimal notation.
PIDs if set will help liberate the broadcast, but you will not see the true name, one audio stream, and leave you with just one channel of any bouquet once searched.
They are also useful for trying to lock into a signal thats just on the threshold of reception.