Flap steak goes by many names, like flap meat, bottom sirloin butt, bavette steak in France, or fraldinha steak in Brazil, and it packs a ton of flavor at an affordable price.
Steak aficionados know that flank, skirt, and hanger steaks are not typically as highly-regarded as expensive cuts, but they're high-quality and affordable for that exact reason.
Flap steak fits into the category of "butcher steaks" — cuts that butchers like to keep for themselves. It also comes from the flank of the cow, just farther back than flank steak.
Flap steak tends to be thin, fibrous, and chewy, so it should be well-marinated, prepped, cooked carefully, and always cut across the grain for maximum tenderness.
You don't want to overcook this cut since it's so thin. Always use a high heat and short cooking time so the outside gets a nice char, but the inside stays juicy and medium-rare.