They're called Camlock Fasteners ... about all they're good for is getting lost!
They look like screws but they're really just a shaft with a cross piece at the end. They're spring loaded and actually aren't suppose to fall out.
They work by pusing them in and turning them until the cross piece locks into a groove. The problem is with time the groove wears down allowing the cross piece to easily slip out (when vibration is introduced at engine start, turbulence, etc.) and then the spring will force the head out and the whole thing hangs loose. With enough vibration and time the thing wears and falls out.
An easy way to make sure they're "locked" is once they are all fastened take the palm of your hand or side of your hande (opposite the thumb) and firmly "bang" or "pound" on the cowling or pannel along the line of fasteners and see if any come loose/pop out.
The only way to trully save a loose camlock is grind a new groove or replace it.
Walk up to any airplane that uses these things and you're bound to find one or two missing.