What is Syndication?
Short answer
Republication of an article across a network of other sites, common with newswires.
A wire placement may appear on dozens or hundreds of sites. Most are automated republications with little readership of their own.
Syndication reach is real for visibility and brand-name searches, and close to worthless for SEO, since the copies are usually canonicalised or nofollowed.
Why it matters
A "300+ sites" headline figure is a syndication count, not an audience. It is worth knowing which one a seller is quoting.