It is a break off screw to make it tamper proof. The bronze thing that your hose is on is called a vacuum breaker. They are code.
The mesh is just a sediment catch. Remove and just use a rubber washer.
I would remove that stupid thing I had 1 on each of my outdoor faucets and removed them both. Both of them acted as auto-drains to empty out the faucet for freeze protection but when you turned off the faucet you got sprayed all over with water.
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