A subagent is a separate AI instance that Claude Code launches to handle a specific task. Each one runs in its own context window with its own tool access. The parent agent kicks it off, and the subagent does the work independently.
I'll show you why this matters: without subagents, a -hour task takes hours. With them, you can split that work across or parallel agents and finish in a fraction of the time. You can also assign specialized agents to specialized roles: one for research, one for writing, one for testing.