This is the 402nd most frequent Vietnamese word.
nghìn
"Ngìn" means "thousand" in English.
The word 'nghìn' (thousand) is used as a numerical quantifier indicating the quantity of money.
Tôi có một nghìn đồng trong ví.
I have one thousand dong in my wallet.
Here, 'nghìn' is used metaphorically to denote a large quantity, in this case, kilometers.
Người đó có vẻ cách xa hàng nghìn cây số.
That person seems to be thousands of kilometers away.
'Ngàn' is a variant of 'nghìn', and here it's used figuratively to express immense gratitude.