Ghost Blocks
A block you placed or broke that is not really there (or is), and the setting that fixes it.
A ghost block happens when the server never confirms a block you placed or broke, so your game shows something the server does not have. You walk into air, or fall through a block you can see.
Ghost Block Fix (Settings, Input) is on by default. When the server never confirms a change, it puts the block back the way the server has it, so what you see matches what is really there.
If you still get them, your connection is probably dropping packets. Check your ping, and try a wired connection instead of Wi-Fi.