They are exactly what the name says. Fouls that technically occur during a live ball, but are enforced as if they occurred during a dead ball. For instance, sideline interference. Even though the interference occurs while the ball is live, it is enforced as if it happened after the play. The play stands, the down counts, and it is enforced from the succeeding spot.
Also, unsportsmanlike conduct fouls no longer fall in this category if they are by players. If a player commits a live ball UNS, it is enforced just like any other live ball foul.