A Service Level Objective (SLO) is like a promise to your users about how well your service or application will perform. For example, if you promise 99.9% uptime, that's your SLO. It's like saying, "Hey, we'll make sure our service is up and running for at least 99.9% of the time, so you can rely on it."
Let's say you're building a chat app. Your SLO could be that messages are delivered within 5 seconds 99.9% of the time. The metric or SLI is the delivery latency. This means that you're setting a target for how fast messages should be sent and received. You then monitor your system to make sure it meets this target.
You can further set a Service Level Agreement by indicating to your users what compensations they'd get if less than 99.9% of their messages are delivered in more than 5 seconds.