This is a phrase used in football to describe the action of an offensive player, usually the quarterback, checking the defense to be sure the offensive play has a solid chance of success. When the offensive team comes out of their huddle and line up at the line of scrimmage, the quarterback will typically wait a moment before assuming their designated position. This allows them to review the formation of the defense and determine if the current play should be executed. If the quarterback sees something in the defense they believe may result in a failed play, they may call a new play or modify the current one, all while at the line of scrimmage.
In the modern NFL, reading the defense is mandatory in order to be a successful quarterback. One of the most successful QB's of all time, Peyton Manning, is considered an expert at reading and analyzing the defense very quickly. In order to combat good quarterbacks, many defenses will now wait until the last moment to reveal their actual defensive positions. In some cases, the defense will read the offense and adjust accordingly if necessary.