Players on a 40-man roster are given three Minor League "options." An option allows that player to be sent to the Minor Leagues ("optioned") without first being subjected to waivers. When a player is optioned to the Minors for a span of more than 20 days, he loses an option.
An option applies to an entire season, meaning that a player can be sent to the Minors and recalled to the Majors any number of times over the course of a season while only losing one option. If he is again optioned to the Minors for 20 or more days in the following season, he would lose his second option and can once again be optioned to the Minors and recalled any number of times that year. The same is true of a player's third option year.
When a player is out of Minor League options, however, it becomes a trickier matter for his club. Out-of-options players are more likely to make a club out of Spring Training, because that player must first be removed from the 40-man roster and exposed to outright waivers before he can be sent outright to a Minor League affiliate. Knowing that they stand a large risk of losing that player, clubs will sometimes carry an out-of-options player early in the year even if that player was outperformed by someone with remaining Minor League options during Spring Training.
If a player misses an entire option year due to injury or expends his third option year before he has completed five professional seasons (Major Leagues and Minor Leagues included), he can receive a fourth option year.
A player's option years do not need to be used in succession. Any player with fewer than five years of Major League service time and an option year remaining can be optioned to the Minor Leagues. Players with more than five years of service time must consent to being optioned.
Mets shortstop Wilmer Flores entered the 2015 season out of options, having been added to the 40-man roster in November 2011. Flores was optioned to the Minor Leagues for the entire 2012 season and spent significant chunks of the 2013 and 2014 seasons in the Minors as well.
Then-Padres outfielder Rymer Liriano missed the entire 2013 season -- an option year -- due to Tommy John surgery and was rewarded with a fourth option year as a result.