How faceted navigation works
A category page lists products. Facets are the controls beside that list which narrow it — size, colour, brand, price band, availability — and each selection a shopper makes usually produces a new URL, most often by adding parameters to the existing address. That is convenient for the shopper and dangerous for search engines, because the number of possible combinations multiplies with every facet you add, and each combination is a crawlable page.
The result is a very large set of addresses that mostly say the same thing in a different order. Search engines have to fetch them, decide which are worth indexing, and work out which of several near-identical pages deserves to rank. Left unmanaged, that work consumes crawl budget that should have gone to product pages, and splits the signals for a category across dozens of thin variants.
Why faceted navigation matters
For a shop with a wide catalogue this is often the largest technical problem on the site, and it is invisible in a normal page count. A shop with a modest number of products can expose an enormous number of URLs, most of which have no search demand behind them and no chance of ranking for anything.
There is an opportunity inside it too. A small number of filter combinations do match real searches — a brand within a category, a colour people search for by name, a size that defines a product type. Those deserve to be proper indexable pages with their own title, heading and copy. Everything else should stay useful for shoppers and invisible to search engines.
Where faceted navigation goes wrong
The commonest failure is doing nothing, then discovering that most of the URLs Google has crawled on the site are filter combinations. The second is over-correcting: blocking the parameters in robots.txt so crawlers cannot fetch them at all, which also stops them reading any canonical tag or noindex instruction those pages carry. A page that is blocked cannot tell a search engine what to do with itself.
Other recurring problems are facets that produce a different URL for the same selection depending on the order the filters were clicked, filters that return an empty result on an indexable address, and a filter menu that links generously into every combination and passes authority there.
How to act on it
Decide deliberately which combinations you want indexed, and treat all the others as a shopper convenience. Give the chosen few clean, stable URLs and content of their own. For the rest, keep them crawlable but non-indexable, canonicalise them to the unfiltered category where the listing is genuinely the same, and stop linking to them from the main navigation. Applying filters without generating a new indexable address at all is often the simplest answer.
Then verify rather than assume. Crawl the site the way a search engine would, read what the server logs and Search Console actually report, and confirm the rules survived the last theme or platform update. This is technical SEO work that repays being rechecked, because a plugin change can undo it silently.