。
Can I get an extra blanket?
Literal
Spare-[appositive-の] blanket [object-を] please.
予備の ('spare, extra') is an attributive use of the noun 予備 ('reserve, backup') — the の marks it as a modifier of 毛布. This construction is everywhere: 予備の鍵 (spare key), 予備のバッテリー (spare battery). ください is the polite-casual 'please give me,' different from ~てください which attaches to verbs.