A website migration is any major change to a site: a new domain, new technology, a rebuilt URL structure. For SEO it's a moment of elevated risk: without a redirect plan you can lose hard-won visibility; with a good plan you usually keep it – and often improve it.
The search engine knows your old addresses – that's where it assigned your rankings and link authority. If, after launch, the old addresses return a 404, that legacy is lost. The core of a safe migration is a 301 redirect map: every old address leads to its new counterpart, not collectively to the homepage.
Beyond redirects, a migration plan covers: keeping the content that earns traffic (a redesign is no reason to delete working articles), reviewing canonicals and metas, updating the sitemap, and monitoring Google Search Console for the first weeks after launch.
For buyers: ask about the redirect plan before signing the contract for a new site, not after launch. A brief wobble in rankings after a migration is normal; a lasting drop means something went wrong – and the cause is easiest to find while it's fresh.