MySQL update+in (what does it mean?!)

[Radgam]

Example for updating thread information in vb:

PHP Code:

$DB_site->query("UPDATE forum SET replycount=replycount+1,lastpost='".time()."',
lastposter='".addslashes($postusername)."' WHERE forumid IN ($foruminfo[parentlist])") 


what does the "IN" mean?

I'm guessing judging by the database it allows you to use a list of numbers so if $foruminfo[parentlist] is 2,1,-1 it will update each one of them?

Thanks, curious for some code mods i'm doing on my forum