in sales, it can mean the difference in higher roi
inbound, is when someone contacts you, who is "already" interested in your domain.
this is very important, because at this point, you the seller, have leverage in negotiations.
virtually all aftermarket platforms like dan, sedo, afternic, unireg, etc., operate on an "incoming" inquiry method.
the majority of all domains sold on daily basis are through aftermarket platforms or via backorder bidding.
outbound, is a term used to make "spamming" or the sending of unsolicited emails to unsuspecting victims, sound better.
when you outbound, you can expect most of your emails to go to junk folder or not get read or they will get ignored.
though it takes lots of work, some do have success with this method.
Inbound (when the buyer contacts you) is a much better position to have because the buyer is already interested and motivated to buy. When you do outbound and contact a potential buyer you have to sell the domain name. This usually ends up with a lower price than if you waited for the right buyer to find you.