‘query_string’ filter hook

Please would someone point me to a place I can find out more about the ‘query_string’ hook, i.e. what is the input and when is it called.

I am trying to write a plugin that amoung other things, can display posts based on other data stored in the wp-postmeta table. The query_string hook could be the way forward, but I can’t find any documentation on it.

Cheers