SSN tax and Medicare tax help

(1)What is SSN tax and medicare TAX for on the W2? are they for retirement purpose? I already paied 403(b) by myself and my employer, I mean I don't want to pay these two extra taxes, it is too much for me, I want to ask my employer to stop withdrawing it from my paystub.

(2)Can I get them back using 1040 return form with the federal withraw together? Thanks. Experts.