Secondary fermentation is racking beer off the primary yeast cake into a second vessel, traditionally to let it clear further and age away from the bulk of the spent yeast — once treated as a near-mandatory step in older homebrewing practice, and now considered optional by most modern brewers for a normal-strength beer.
Why the "always rack to secondary" advice has softened over time
Modern, well-flocculating yeast strains and generally improved homebrew process control mean most standard-strength beers clean up and clear reasonably well left in a single vessel through fermentation and a short conditioning period, without the extra transfer step (and extra oxidation and infection risk that step introduces) secondary fermentation traditionally involved. A lot of current homebrewing advice treats a single-vessel approach as the sensible default rather than secondary racking as the assumed standard.
Where it genuinely still earns its place
Long-aging strong styles — Barleywine, Russian Imperial Stout, Belgian Dark Strong Ale — are the clearest remaining case for a real secondary racking, since getting the beer off a large primary yeast cake before autolysis risk builds up over months of aging is a genuine, checkable benefit that outweighs the transfer risk for a beer already committed to a long timeline anyway.