Steam Family Sharing now requires a two-factor authorization process
Users must now authorize both the machines and accounts they want to share games with.
Previously, if you wanted to share your library of Steam games with someone, you needed only to authorize another computer that would be able to access it. Now, a friend or family you wish to share with needs an authorized computer but also a specific, authorized Steam account. You may authorize up to 10 computers and 10 accounts that share your library at any given time.
The policy is marginally more restrictive, but Valve says it will give lenders more control while reducing the risk of bans resulting from random users accessing and abusing an authorized machine. The security benefit, in short, is that if an authorized Steam account on an authorized computer is password protected, there is a better chance only the person you intended to share with will access your games.
Steam Family Sharing, allows friends and family to share their libraries of Steam games. The feature will be supported on PC, Mac, and Linux and is currently in beta.