Tree radical on the left - also 柏's indexing radical. Anchors the meaning directly: 柏 is the cypress tree, a hardy evergreen planted at temples and graves. Same radical family as 松 (pine), 柳 (willow), 桃 (peach) - every species name lives here.
Right side 白 supplies the sound: bái → bǎi with only a tone shift. Same phonetic appears in 拍, 怕, 帕, 迫. A faint semantic hint may also be present - the cypress's pale, silvery wood - but the primary role is sound.
