A yeast starter is a small batch of wort — typically well under a litre to a couple of litres — fermented in advance of brew day specifically to grow a liquid yeast culture up to a healthy, appropriately-sized population before it's pitched into the full batch, rather than pitching a single vial or pack directly and hoping it's enough on its own.
Why liquid yeast needs this and dry yeast usually doesn't
Liquid yeast packs and vials generally ship with a smaller cell count than a full-size batch at normal-to-high gravity needs for a healthy fermentation, especially once the pack's manufacture date is more than a few weeks old and viability has started to decline — a starter grows that count back up before pitching. Dry yeast packets, by contrast, are manufactured at a much higher cell density and generally don't need a starter at all for a standard-strength batch, just proper rehydration.
Underpitching is the single most common cause of fermentation problems this site's style pages warn about
Every high-gravity style covered on this site — Doppelbock, Russian Imperial Stout, Belgian strong ales, Barleywine, Traditional Mead — repeats some version of the same warning about adequate pitch rate, and a correctly-sized starter (sometimes stepped up in stages for very high gravity) is the direct, practical way a homebrewer actually delivers on that advice rather than just being told about it.