。
My mother made me a pure white dress.
Literal
Mother [topic-は] me [for-に] pure-white dress [object-を] made-[benefactive-てくれた]
真っ白い = 'pure/bright white' — 真っ (まっ) is an intensifying prefix: 真っ赤 (bright red), 真っ暗 (pitch dark), 真っ白 (pure white). The prefix doubles the first consonant of the following word.