I think what it is is when a contact is offline you can post a messege to him/her and send it, then when your contact next comes online, he/she will be able to view the message and same the other way round. You can do it on Yahoo Messenger and its a bit like the come-back feature in plus but you don't have to be online for your contact to view the message and again the same the other way round.
Hope that helped
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Sizedude