A player is only listed if both are true. Playing well: he averaged at least a useful weekly starter's points (the same line the start/sit verdict uses for his position) over his last 3 games, and played in one of his team's last two, without fading well below his own earlier average. Soft matchup: this week's opponent has allowed at least 12% more fantasy points than the league average to players in his role, over at least 3 games.
The gem score (0-100) is half matchup softness and half recent form, so a player who is only fine but in a dream matchup doesn't outrank one who is both. Injuries then move it: down for his own Questionable tag, a hurt starting QB or two or more offensive linemen out; up for hurt teammates whose work he may absorb and for starters missing on the defense he faces. “Star” means a player already ranked in the top 12 QB, 20 RB, 28 WR or 10 TE on the season by points per game, so a starter in most leagues. Those are hidden by default, which leaves players outside that range who are heating up.
The catch: a few games is a small sample for both the player and the defense, and a defense can look soft because of who it has already played. Check the injury chip, and open the player to see the full matchup table behind the grade.