Handlebar flare refers to how much the lower drop sections of your handlebars angle outward compared to the hoods position. On gravel bikes, this outward sweep improves stability and control when you're riding in the drops on rough terrain, since it effectively widens your hand stance.
For most gravel riders, 8–16° of flare is the sweet spot. This range gives you noticeably better control in the drops without making the hoods position feel excessively wide or awkward. If you primarily ride smoother gravel roads and spend most of your time on the hoods, something closer to 0–5° (essentially a road-style bar) may feel more natural.
Riders tackling technical, loose, or steep off-road terrain often prefer 16–25° of flare for maximum leverage and stability. Just keep in mind that more flare isn't always better—excessive flare can make the hoods feel too far apart and less comfortable on longer rides. Hand position preference is highly personal, so test-riding different flared bars is the best way to find what works for you.