For regulations surrounding kiai time, see: Ranking Criteria
Kiai Time (or kiai) is a term given for a "special" timing section to help emphasize a part of a beatmap. Players can tell if a part of a beatmap has a kiai timing section if they see added visual effects in the background (e.g., flashing beats, flying stars, and fountains).
While kiai is active, all hit objects will flash at the rhythm of the main beat and stars will explode from the cursor when a hit circle is hit. Otherwise, they will fall from the cursor when a slider or spinner is followed.
Kiai time is most commonly used for the "strongest part of a song" which is usually the chorus. Since kiai sections are the strongest part of a song, they are typically slightly more challenging than the rest of the beatmap to follow the song.