Most engineering teams know they should care about reliability. But when it comes to defining what "reliable" actually means, things get fuzzy fast. According to a 2023 report by Xurrent , 74% of businesses struggle to clearly define and communicate SLAs. And that's just the external contract. SLOs and SLIs, the internal targets and measurements that SLAs depend on, often get conflated, skipped, or treated as interchangeable. That confusion has real consequences. Teams miss degradation before users notice. Reliability becomes a feeling instead of a number. And when something breaks, there's no clear signal it was coming. SLIs, SLOs, and SLAs are not synonyms. They're three distinct layers of a system designed to make reliability measurable, manageable, and trustworthy. This guide breaks down each one, shows how they connect, and explains why load testing is what makes all three credible. TL;DR: SLIs measure actual service performance. SLOs set internal targets for those measurements.…