The NFL typically holds a supplemental draft in the second week of July. Often enough, no player ends up being selected, but usually, at least one player is taken. Since 1977, there has been at least one player selected in the supplemental draft 29 times, including 2019 most recently, with the Arizona Cardinals selecting safety Jalen Thompson.
The supplemental draft is used to accommodate players who between the window for declaring for the traditional NFL Draft and the potential supplemental draft experience circumstances that change their status and result in their deciding that they want to exit college and pursue an NFL career.