A Service Level Indicator (SLI) is a metric that tells you how well your service is performing. The metric could be anything, response time, error rate, throughput, or any metric that is important to your application.
For example, let's say you have a web application and you want to know how quickly it responds to user requests. You can set up an SLI to measure the response time of your application. If the response time is consistently fast, your SLI will show a good performance grade. But if the response time starts to slow down, your SLI will reflect that, giving you a heads-up that something might be wrong.
You can use SLIs to create alerts that notify you when your service isn't meeting its performance goals. Or you can define Service Level Objectives (SLO) that tells you how well you're doing with the metric over time, or Service Level Agreements (SLA) to indicate to your users the minimum reliability they can expect from your application.