Triggers and SLOs are both used to notify people about things that are happening based on some telemetry fields.
Triggers are typically for a known, historical problem like disk space depletion or pod failures. They are easy to build based on Metrics or Logs datasets to alert people when certain thresholds are met.
SLOs are typically for unknown problems, where there's a desired performance level that should be achieved. The SLO is intended to catch "slowness" or "elevated errors" before it turns into a cascading failure. The SLO Details page shows a bubble up on showing where SLO failures are highlighted and compared so people can jump directly to the slow or error spans.
More information can be found on this docs page: https://docs.honeycomb.io/get-started/best-practices/alerts/#guidelines-for-when-to-use-slos-and-trigger-alerts