Annotation queries return events that can be visualized as event markers in graphs across the dashboard.
For example, node_boot_time_seconds{job=~"$job",instance=~"$instance"}*1000 > $__from < $__to can detect reboot events and automatically annotate all graphs.
Manual Annotations
Manual annotations are stored per dashboard in Grafana’s database, and queried from Annotations & Alerts (Built-in) when displaying the original dashboard.