Rule 5 Primer
The basic goal of the Rule 5 Draft is to prevent teams from unduly stockpiling talent in the minors without giving the players a shot at the majors. The timelines for Rule 5 eligibility are complex, but as a basic rule of thumb, high school draftees and international free agents (IFA) are first eligible four years after signing, with college players being eligible three years after signing. And as with Elvis Luciano, any player who signs a second contract (for whatever reason) becomes immediately eligible.
For 2021, this generally means HS draftees and IFAs signed in 2017 and college players signed in 2018. In addition, all players who were previously Rule 5 eligible remain eligible, unless they are likewise added. This includes minor league free agents signed prior to the Rule 5 draft and former major leaguers who have been outrighted off the 40-man roster.
In the major league phase, teams can select eligible players from other organizations who are not on a 40-man roster for $100,000. Drafted players have to remain on the 26-man (active) roster for the entire subsequent season, with a minimum of 90 active days, or be put on waivers and offered back to the original team if cleared.
If the player fails to reach 90 active days, he has to remain on the active roster in subsequent seasons until he reaches 90 active days. One other note is that a selected player cannot be removed from the 40-man over the winter. The minor league phases of the draft are much less important and generally used just to fill out rosters, with no similar roster restrictions.