Steam Gifts is a system set up in Steam to allow you to trade games with other users of the service, therefore both getting a game that you want and everyone's happy.
Unfortunately there's been a nasty little bug in the system that scammers have been exploiting to get games for nothing. Simply if the payment is declined for the game you traded for, you lose that game not the scammer while the scammer makes off with the game you traded to them and paid for. Basically it's not good and so Valve have changed the rules a little bit. Now any game in your inventory set aside for trading can only be traded 30 days after purchase, which gives plenty of time for checks to be sure the payment method won't be declined. This also means that legitimate trades that get hit with this payment declined issue will no longer happen either, which is good for all.
Valve said: “We’ve made this change to make trading gifts a better experience for those receiving the gifts. We’re hoping this lowers the number of people who trade for a game only to have the game revoked later due to issues with the purchaser’s payment method.”
The rule only applies to trading though, so if you want to buy and gift a game to a friend of family member for the holidays this new rule won't be there to trip you up and stop you sending them your gift.