Epics
A piece of work too big for one task, with its children underneath it.
Use an epic for work too big for one task. File tasks underneath it and the epic shows how far they have got.
File a task under an epic
Open the task and pick an epic. Only a task in the same project can go under one, and only a top-level task, because a subtask already belongs to its parent.
Read progress
Progress comes from the children: how many are finished and how many points they carry. An epic with no children has nothing to report and says so.
Filter by an epic
Epics are a filter on any board, so “everything under this epic” is one click from the filter row.
Common questions
Can an epic sit under another epic?
No. One level, so a plan never becomes a tree nobody can read.
Why can I not pick an epic for this task?
It is in no project, or in a different one. An epic belongs to a project, so its children must too.