Since I got the job, I’ve lost interest.
Every morning Koichi is late for work because he hasn't decided on a hairstyle yet.
"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.