The 15-day injured list allows clubs to remove injured pitchers or two-way players (position players who are eligible to pitch in any circumstance) from the 26-man roster (it was 25, prior to 2020) while keeping them on the 40-man roster. Pitchers or two-way players can be placed on the 15-day injured list for any type of injury, and must remain out of action for at least 15 days, though the player can also stay on the list for considerably longer than 15 days, if necessary.
Players may be placed on the 15-day injured list "retroactively," meaning the stint is backdated to the day after the last date on which the player appeared in a game. For instance, if a pitcher was removed from a start due to shoulder pain on June 1, he could be placed on the 15-day injured list on June 3, retroactive to June 2. In that case, he would be eligible to return from the injured list on June 17.
The injured list used to be 15 days for all players until the 2017 season, when it was reduced to 10 days. Position players will still have a 10-day IL under the new rules.