The play-in is the round before the NBA playoffs in which the teams ranked 7th through 10th in each conference battle for the last two spots in the playoff bracket. It was introduced experimentally in 2020 and became a permanent format in 2022.
The play-in first appeared in the 2019-20 season, during the return to play in the Orlando "bubble" after the pandemic pause. Its first version was very limited: it only activated between the 8th and 9th seeds of each conference, and only if the 9th seed finished the regular season within four games of the 8th. The NBA expanded the format in November 2020 for the 2020-21 season, now including the 7th through 10th seeds of each conference, and on July 22, 2022 the league made it a permanent, fixed format every season, with no games-back condition anymore.
At the end of the regular season, the 7th through 10th seeds of each conference play three games among themselves instead of qualifying directly. The 7th seed hosts the 8th: the winner takes the 7th seed spot in the playoffs outright. The 9th seed hosts the 10th: the loser is eliminated from any playoff chance that season. The loser of the 7th-8th game then faces the winner of the 9th-10th game, and the winner of that final matchup takes the 8th seed spot. In total, of the four teams that enter the play-in in each conference, only two end up making the playoffs.
The play-in forces teams to genuinely fight until the last regular-season game to avoid the bottom tier of the standings, something that used to ease off much earlier under the old system of qualifying directly up to 8th place. For a team, the difference between finishing 6th (direct qualification) and finishing 9th (two elimination games just to survive) is huge, which adds tension and uncertainty to the final weeks of the regular season, right when interest used to fade for mid-table teams.
If your Match Prophets league follows the NBA, the play-in stage tends to be one of the most entertaining to predict: these are win-or-go-home games between evenly matched teams, with far more uncertainty than a normal playoff matchup. Every game scores the same as any other: 3 points for the exact score, 2 for picking the winner.