A flat mount (or drilled mount) means your bindings are screwed directly into the ski's top sheet—no tracks, no rails, no proprietary interfaces. It's the most common mounting method out there, used on the vast majority of alpine and touring bindings.
The big advantage is performance: flat mounting gives you the lightest, most direct connection to your ski. There's no extra hardware between you and the snow, so power transfer feels immediate and precise. That's why racers, freeriders, and touring skiers overwhelmingly prefer this setup.
The trade-off is flexibility. Once drilled, your binding position is fixed—if you change boot sole lengths or want to shift your mount point, you'll need a remount (new holes). Also, drilling must be done precisely, ideally by a shop with the right jig and depth stops. Too close to existing holes weakens the ski, and going too deep can damage the core.
For most skiers who aren't swapping bindings between skis or sharing setups, flat mount is the way to go.