It's normal for floorboards to have cuts in them as getting boards long enough for the entire span is impractical or would be more costly. Shorter boards are commonly used so you will have joints.
However, the joints are normally level and both sides of the board should be supported. A joint should only be made on top of a joist so that both sides are properly supported. In your case it seems like the top half doesn't have the proper support. Perhaps the joint missed the underlying joist and the builder was too lazy to fix it properly or maybe some extra bracing has come loose.
The fix is going to be to remove the board on top and add some bracing underneath it. It should be pretty straightforward to screw a block of bracing to the nearby joist. If the joist is more than 1" away, you may just want to remove the top and bottom board and trim them properly do the joint is centered on the joist or replace the boards if trimming them would make them too short.