Stars shower the screen indicating Kiai Time has started.
Kiai Time shows in the upper-left of the screen when active during edit mode.
Kiai Time is defined by the mapper on a beatmap between 2 Timing Sections. You can recognize a Kiai time because the screen flashes and you will see a star fountain, afterwards 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 or will fall from the cursor when you follow a slider or a spinner.
The most common term for Kiai Time when editing beatmaps is the "strongest part of the song" this is normally the chorus of the song, however some songs don't have a chorus, but they do have a strong part for you that feels cool, so you could insert Kiai Time in that section. Be advised however that spamming kiai start sections will cause a spam in kiai fountains. That is unrankable, and mostly hated by players because of the lag it may cause.