A service-level objective is a rule that sets the standards for how a service should work. Imagine you're playing a game with your friends, and you all agree that you want to hit a target with a ball. Your service-level objective is to hit the target three times out of every five tries.
Now, let's say you're a grown-up working for a company that provides internet service. Your company promises that your internet will work for at least 99% of the time, which means they assure that your internet won't go down for more than 1% of the time in a month. That is an example of a service-level objective.
So, a service-level objective is like a promise that a company makes to its customers. It sets specific goals on how well they will perform and ensures that they deliver a high-quality service reliably. Setting a service-level objective helps a company to be accountable and keep their customers happy.