This is the 496th most frequent Vietnamese word.
lá
"Leaf."
Here, 'lá' refers to the foliage or leaves of trees.
Lá cây rụng vào mùa thu.
Leaves fall in autumn.
In this context, 'lá' is used as a counter word for letters or written correspondences in Vietnamese.
Anh ấy viết thư lá thư tình.
He writes a loving letter.
Here, 'lá' specifically refers to a large leaf, namely a banana leaf.