one bag; one sack
Counts one bag or sack of something. Often used with the counter 袋 (ふくろ) for bagged items.
I bought one bag of rice.
Please give me one bag of snacks.
一箱 means 'one box', while 一袋 specifically refers to a bag or sack.
Compound of 一 (ひと, 'one') and 袋 (ふくろ, 'bag').