A sub-agent is a smaller AI helper asked to handle one part of a larger task. A main agent can split work into pieces, ask sub-agents to investigate them, and combine their results.
For example, one helper could check a set of files while another looks for related documentation. The main helper then reviews both reports before suggesting what to do.
Sub-agents can help separate work, but they may duplicate effort or return conflicting answers. The main agent still needs to check their work, and a person should review important outcomes.