They are arcade sticks that, instead of using a joystick with 4-directional switches, use 4 buttons for the directions. They are often called “Hitbox” because that’s the brand that popularized them. Some people also call them “all-button controller” or “stickless controller”. The advantages are ergonomics (many people have developed carpal tunnel syndrome from traditional joysticks) and economy of motion (it’s easier to press 4 buttons with 4 fingers than move the entire wrist/forearm to move a joystick).
It’s pretty much like using a keyboard for fighting games, but you only have the buttons that you actually use, the keys are large buttons, and there are no issues with pressing many simultaneous buttons (many keyboards have problems with that). People also often use PCBs that are compatible with PS5 or other consoles, and also SOCD cleaners (which is a piece that helps configure which inputs should be sent if you press “Simultaneous Opposite Cardinal Directions”).
They’ve been gaining more and more popularity over the last few years. If you are good at typing, you may be more comfortable playing fighting games with this than with a traditional joystick or even a gamepad.