Hydration attaches JavaScript event handlers to server-rendered HTML.
The process:
Browser receives HTML from server
User sees content immediately
JavaScript downloads and executes
Framework attaches event listeners
Page becomes interactive
Problems:
- Hydration mismatch: Server and client render different content
- Time to interactive delayed until hydration completes
- Duplicate work: Server renders, then client re-renders
Interview tip: Mention hydration when discussing SSR. It's a common pain point.