When a club "non-tenders" a player, it declines to give that player a contract for the upcoming season, thereby immediately making him a free agent. Players on the 40-man roster with fewer than six years of Major League service time must be tendered contracts each offseason by a set deadline -- typically a date in early December -- or non-tendered and released to the free-agent pool.
In many instances, a club will non-tender a player because it feels the raise he will receive in arbitration would be greater than his on-field value. In other cases, a club will non-tender a player simply to clear a spot on the 40-man roster -- even if that player isn't due much more than the league minimum the following season.
Henderson Alvarez was due to receive $4 million or more in arbitration following the 2015 campaign, in which he made just four starts before undergoing season-ending shoulder surgery. Rather than tender a contract to a player who missed considerable time, the Marlins non-tendered Alvarez in December 2015.
Also in December 2015, the Astros elected to non-tender first baseman Chris Carter rather than pay him a raise on his $4.175 million salary from the prior season.