PCR (Program Clock Reference) is the timing heartbeat of an MPEG transport stream. The encoder periodically inserts a snapshot of its 27 MHz system clock into the stream, and the decoder locks its own clock to those samples, recovering the exact timebase the content was created on. This matters because the PTS and DTS timestamps that schedule each audio and video frame are only meaningful relative to that shared clock — without PCR the decoder would slowly drift and audio and video would separate. PCR is delivered frequently and accurately, because jitter in its values becomes jitter in playback timing and ultimately lip-sync error.
