Short answer: An SLA (service level agreement) is the provider's written uptime promise, commonly 99.9% or 99.99% a month, and the service credits it pays when it misses that target. Credits rarely cover the real cost of downtime, so treat the SLA as a signal of how seriously the host engineers availability.
SLA (service level agreement) explained
A 99.9% monthly SLA allows about 43 minutes of downtime, 99.95% about 22 minutes and 99.99% about 4 minutes. Credits are usually a percentage of the monthly fee, must be claimed within a deadline and exclude scheduled maintenance and attacks.
Several budget hosts publish no SLA at all. The provider pages on this site note each host's published SLA, or its absence.
See it in the data
Frequently asked questions
What is a good SLA for a VPS?
99.9% is common; 99.95–99.99% suggests redundant power, network and storage. Check what counts as downtime and how credits are claimed.