Handing a bounded piece of work to a fresh instance with its own clean context, which reports back a result. The main benefit is context isolation: the parent never has to hold all the detail of the sub-task, only its conclusion.
Technical view
Each sub-agent receives a scoped task and returns a structured result, keeping intermediate exploration out of the parent context and enabling parallel execution across independent sub-tasks. Costs are token duplication from repeated context setup, latency where results must be gathered before proceeding, and loss of information the parent might have needed. Sub-agents should have narrower tool access than the parent.
Related
Part of AI Terminology Index, section: Agents and Tools.