They complement each other.

Subagents are mainly a token context optimization hack. They're a way for Claude Code to run a bunch of extra tools calls (e.g. to investigate the source of a bug) without consuming many tokens in the parent agent loop - the subagent gets its own loop, can use up to ~240,000 tokens exploring a problem and can then reply back up to the parent agent with a short description of what it did or what it figured out.

A subagent might use one or more skills as part of running.

A skill might advise Claude Code on how best to use subagents to solve a problem.