subqueries in BLIND SQL

i was reading a BLIND SQL pdf and it mentioned subqueries what is this and how do you insert it??

Here this touches on it: http://www.hellboundhackers.org/articles/345-blind-sql-injection.html

Another url for everyones benefit: http://www.justfuckinggoogleit.com/

JUST FUCKING GOOGLE IT

yea because i totally didn't use Google to find the blind sql pdf document

your right I'll just try the same thing again

and as for the article, i got lost at sub queries which is why i posted this because i Googled sub queries as well and got nothing