Sellers sometimes decide to make their items open to offers. This means that you can put in a bid to an amount that you think you would like to pay for an item. If you put in a reasonable offer, the seller is more than likely going to accept it.
I guess, occasionally, its worth putting in a silly offer.... if you want to chance your luck - however you are more than likely not going to have it accepted. You can, of course, "up" your offer if you are refused. If you mess about with offers with the seller they may block you to prevent you from bidding on their items, so if you really want the item.. my advice would be to put in a realistic bid.
If you wonder how sometimes the seller seems to respond immediately to your offer, its because when the seller has set up the sale they have put in an automatic minimum amount they will accept for the item - so the system is sometimes responding to you - not the seller!
When you confirm your amount you are entering into the process of actually buying the item. If the seller decides to accept your offer it is the same as any winning bid on eBay and you are obliged to pay for the item... so again ensure that you do want to bid, you do want the item and you know exactly what you want to pay for it.