To Be Settled
Intransitive verb and Godan verb
きまる
仕事が決まってからは、きょうみがなくなりました。
Since I got the job, I’ve lost interest.
コウイチは毎朝かみがたが決まらないせいで会社にちこくする。
Every morning Koichi is late for work because he hasn't decided on a hairstyle yet.
「それは本当にいい考えだね。そうしよう。」「よし、じゃあ、それで話は決まりだ。」
"That’s a really good idea. Let’s do that." "Okay, then it’s been decided."
You've seen the word 決める before, hopefully. This is similar, except for instead of doing the action of deciding, something has just been decided (by something or someone else). You are not the decision maker, here. That's why this word is to be decided.
The reading is the same as 決める, so make sure you know that word so you can apply that knowledge over here.