Recently, Valve introduced some security updates that put holds on trades if you did not make efforts to secure your Steam account. Today, Valve issued an update to those changes based on collected data and suggestions.

These new changes will take effect on March 9, 2016.
Below are the changes that will take place on March 9th. If you are already protected by the Steam Guard Mobile Authenticator

If any of these new changes confuse you, I strongly urge you to read through Valve's full posting on these changes. They go into detail about why these changes are being implemented.