This is the 3467th most frequent German word.
knall
"Knall" means "bang," "pop," or "crack" (a loud, sharp noise).
In this sentence, 'Knall' is a noun referring to a loud, explosive sound.
Plötzlich hörten wir einen lauten Knall.
Suddenly, we heard a loud bang.
Here, 'knall' is used as a prefix emphasizing the intensity of red in 'knallrot'.
Der knallrote Mantel fiel jedem sofort auf.
The bright red coat was immediately noticeable to everyone.
Here, 'knallte' is a verb form referring to the sound made by the action, derived from 'knallen'.