Minor League Options
Normally, a player on an MLB Reserve List (40-man Roster) is allotted three minor league option years. This means the player can be sent to the minors and be available 24/7 for recall back to the major league club whenever the player might be needed during the course of three different seasons. Note that if a player spends 20 or more days of an MLB regular season on Optional Assignment to the minors, an option year is expended, but if the player spends 19 or fewer days of an MLB regular season on Optional Assignment, an option year is NOT expended.
Also, if a player with minor league options remaining is sent outright to the minors prior to MLB Opening Day and then is reacquired by the same club and added to the club's MLB Reserve List (40-man roster) during the MLB regular season and at least 20 days after MLB Opening Day, or if a player with minor league options remaining is sent outright to the minors during the MLB regular season and then is reacquired by the same club during that same MLB regular season and added to the club's MLB Reserve List at least 20 days after the Outright Assignment, an option year is spent, even though the player was not optioned to the minors.
After three minor league option years have been spent, the player is said to be "out of options" and the only way the club can send the player to the minors is by removing the player from the 40-man roster and sending him "outright" to the minors. And the club can do this only after first securing Outright Assignment Waivers (which are irrevocable) from the other 29 MLB clubs.