What is SDL based idea?

i know that SDL package usages are a lot and one this uses is
synchronization between video and audio
what is the idea to achieve that.
for example:
SDL may do synchronization as following:
it diplays some video frames then out a sample of audio to the sound card
using semaphores.

is right or wrong ?
plz some explain or send me good link on how to make synchronization between
video and audio.
thnkx