Flat-mount is the newer caliper attachment standard for hydraulic disc brakes, where the caliper bolts flush against the frame or fork rather than sitting on raised posts. It's become the go-to standard on most modern gravel bikes because it's sleeker, slightly lighter, and simply looks cleaner.
Performance-wise, flat-mount hydraulic brakes deliver the same excellent stopping power and modulation as post-mount hydraulic systems — the difference is really just the mounting interface. You get consistent, powerful braking in wet, dry, and muddy conditions, which is exactly what you need when you're descending loose gravel roads.
The main thing to know is that flat-mount is the current standard across gravel and road bikes, so finding replacement parts and upgrading down the line is straightforward. If you're shopping for a new gravel bike, flat-mount hydraulic disc brakes should be your default expectation — they're what most quality builds come with.