A service level agreement, or SLA, is a provider's written commitment about the level of service it will deliver, most often a percentage of uptime over a month. If the provider falls short, the agreement usually offers service credits, which are a discount on future bills, as the remedy.
An SLA gives you a clear benchmark for comparing providers and plans, and it states how downtime is measured and how to claim credits. Higher uptime figures usually correspond to configurations that cost more, such as setups with redundancy.
SLAs have limits worth understanding. They often apply only to specific configurations, may exclude planned maintenance and certain causes, and require you to file a claim within a deadline. Credits are generally a partial refund, not compensation for lost business or data. A promised percentage is also a target backed by a remedy, not a guarantee that an outage will not happen.
To use it properly, read the exact terms for the product and configuration you plan to use, note any conditions such as running across multiple zones, and compare the SLA with what your application actually needs. Keep your own monitoring so you can document outages if you need to claim a credit.