The SLA, SLO, and SLI are related concepts though they're different concepts.
So here is the relationship. The service provider needs to collect metrics based on SLI, define thresholds of metrics based on SLO, and monitor the thresholds of metrics so that it won't break SLA. In practical, the SLIs are the metrics in the monitoring system; the SLOs are alerting rules, and the SLAs are the numbers of the monitoring metrics applying to the SLOs.
Usually the SLO and the SLA are similar while the SLO is looser than SLA. The SLOs are generally used for internal only, and the SLAs are for external. If a service availability violates the SLO, operations need to react quickly to avoid it breaking SLA, otherwise, the company might need to refund some money to customers.
The SLA, SLO, and SLI are based on such assumption that is the service will not be available 100%. Instead, we guarantee that the system will be available greater than a certain number, for example, 99.5%.