A span is defined as the duration of a single event. Spans are automatically captured by the APM agents, but you can also define custom spans. Each span has a type and is defined by a different colour in the Timeline visualization (also known as the waterfall).

Example view of transactions detail page in the APM UI in Kibana

You can view a span in detail by clicking it in the Timeline. This displays the recorded SQL database query or source code (in-app frames) around the event that was captured.

Example view of a span detail in the APM UI in Kibana