This is the 62nd most frequent Vietnamese word.
các
"Various" or "plural marker."
In this sentence, 'các' is used to denote the plural form of 'bạn học sinh' (students).
Các bạn học sinh đang chơi trong sân trường.
The students are playing in the schoolyard.
Here, 'các' emphasizes the plural nature of 'món ăn' (dishes).
Các món ăn trong thực đơn hôm nay rất ngon.
The dishes on today's menu are very delicious.
In this context, 'các' is used to indicate plurality and inclusivity of the 'loài chim' (bird species).