Seized, locked up. Metal fused to metal or something preventing something from moving. In an engine, metal to metal contact with movement will generate heat high enough to weld it together. Or, something else is preventing the movement. burs or rust, etc.
My engine has been sitting unused for several years and does the same thing yours does. Starter spins but flywheel does not.
I tasked myself to try to get it to move before month is out. After this, I will see if you still have the engine available for parts. If your engine was running recently, I may have a better shot of getting yours started rather than mine.
I live on Long Island so if it comes to it, I can pick up.