See also: How to manage action tasks
In Juju, a task is the execution of an action (via juju run) or of other arbitrary scripts (via juju exec) on a target unit.
Action tasks are run as defined by the charm author (default: sequentially), whereas tasks related to other scripts are run as set by the charm user (default: parallel).
A group of tasks queued by running an action across one or more units forms an operation.