Dahon Folding bike, seat came right off!
Looks like there's something missing.  The golden piece should go around the top of the black piece of the frame, and should clamp it tight without the two edges touching by the threadded bolt.

The silver piece looks like a shim or a spacer of some sort, and the seat and seatpole/post has not been pictured, but would go inside the silver tube.

Is there a second seat clamp (golden piece) missing, to go around the silver bit to hold it to the seat post proper ?

Edit:-
A google suggests these have telescopic seat posts, so silver piece is not just a static shim.   The other possibility is your seat clamp may have stretched and is no longer providing enough clamping force.