A sub-task is a smaller piece of work that belongs to a parent issue, used to split a story or task into steps that can be assigned and tracked separately.
Sub-tasks make progress on a larger item visible and let several people work on it at once. A story like "Export invoices as PDF" might have sub-tasks for the design, the backend, tests and documentation.
Sub-tasks vs checklists
Use sub-tasks when steps need their own assignee, status or estimate. Use a checklist for quick steps one person will tick off.
Sub-tasks in Starlex
Any issue can have sub-tasks, and issues also support checklists. See issue tracking.