When you spend time or money on something risky, you hope that there will be a good result. If so, you say that it "paid off". For example, the father of a professional musician might say:

I'm glad to see that all those years of piano lessons I put him through finally paid off.

You can talk about investments and projects at work with this phrase:

We're spending over fifty thousand dollars on these improvements. That may seem like a lot of money at first, but I'm confident that it will pay off down the road.

One other example of a situation you can use "pay off" in is when talking about trusting someone who might not be very trustworthy:

My trust in her paid off in the end, though. She ended up doing the right thing.