>>3 Sage means you don't want to bump the thread up to the top. You check the box that says "Sage" at the top right corner of the textbox where you reply/comment so the thread isn't brought up to the top.
You should read the thread called "Read this if you're new" on the Main board; it'll explain all these things and you can post other questions you have in the FAQs thread as well.
So basically, if you want to reply to me now, check the "Sage" box so you don't bring this thread up over other ones that are actually important :)
/sage (means I'm letting others know I've saged this reply)