counter
counter for thin, flat things
Used for counting thin, flat objects such as petals, leaves, sheets of paper, or slices. The reading ひら is specific to this counter usage.
桜の花びらが三枚、風に舞っている。
Three cherry blossom petals are dancing in the wind.
この書類を一枚ずつ配ってください。
Please distribute these documents one sheet at a time.
The more common reading for the counter 枚, used for counting flat objects in general. ひら is a less common alternative reading with the same meaning.
The kanji 枚 originally meant 'a flat piece' and is used as a counter for thin, flat objects. The reading ひら is a native Japanese reading (kun'yomi) of the kanji.