This idiom means to eat.
Though it is short it is used very much in everyday English.It is basically used by Chinese people.A chow is a breed of Chinese dog.It is also used generally in military as a slang. It refers to people to eat.
1) "Chow down, Madam" said a Chinese server in a brown coat.
2)Mahesh is always ready to chow down at lunch time.
3)Karthik always behaves as if he has been deprived of food and always sits to chow down food at any time.
4)"Now be quiet and chow down your food", ordered Peter's mother.