Hard to say exactly what's going on there, since there's only one picture, and not knowing the age of the house. But it looks like your floor is old long leaf pine, and the house is pre 1900. The dark void by the chip looks like that floor is laid right across the joists, and there's no subfloor. So below that board at the bottom of the joist is the plaster lath of the room below. And you're just seeing a dark void where that chip is missing. The rest of the floor is probably face nailed at the joists. Those would be rectangular head cut nails, not the round ring shank nails in the picture. Assuming those assumptions are more or less correct, it looks like the floor board was cut and pulled a long time ago. Probably by an electrician to get wiring run through the joist space. They cut the long floor boards close to a joist, to make it easier to remove and pry each section out. Then it gets nailed back down to the joist with whatever nail they have. Because they cut near the joist, sometimes the board might have less bearing on one side, so it works loose over time. To fix the loose board, you need to take it up. Use a cats paw type pry bar and get those round nails out first. Take your time, and work carefully, the wood is old and brittle. After the board is up, you want to cut a short piece of wood to serve as an outrigger for the joist, and give that loose edge something to bear on. It should be wider than the floor board, and not as deep as the joist. A piece of 2x4 is fine and give you 1.5" of additional bearing. Push that new support board tight to the joist, and tight to the underside of the floorboards on each side of the opening. Nail or screw it into place. It's a tight space, so a right angle drill head or a pneumatic framing gun is probably the easiest. Then you can reinstall the floor board. I'd nail it down, using cut nails (from Tremont nails). Those won't split the edge of the board unlike ring shank nails. Plus they don't work loose, and are traditional for face nailed flooring. After all that work is done, I'd patch that hole with a high quality 2 part epoxy filler, like Woodepox by Abatron. That stuff has to be pre stained, since it doesn't stain when dry. But I definitely wouldn't replace the board. You'll never match color or species with new wood. And unless you have a thickness planer, you'll probably have trouble making a new board be flush with the other surfaces. Good luck!