This is the 3816th most frequent Swedish word.
kastat
"Kastat" means "thrown" in English.
Here, 'kastat' is used in its perfect form as past participle and indicates that an action has been completed with respect to the ball being thrown.
Jag har kastat bollen.
I have thrown the ball.
In this usage, 'kastat' also serves as a past participle, with the nuance of the action referring to discarding or disposing of the letter.
Han säger att han har kastat brevet.
He says that he has thrown the letter away.
'Kastat' here, although it literally means 'thrown', is used idiomatically in the phrase 'kastat upp', which means 'vomited' in Swedish.