This is the 427th most frequent Hindi word.
कस
Tightness or firmness.
Here, 'कस' is used as a verb meaning to tighten or fasten.
उसने जूतों के फीते कस लिए।
He tightened the shoe laces.
In this sentence, 'कस के' is used as an adverbial phrase meaning 'diligently' or 'intensely'.
कस के मेहनत करो, सफलता मिल जाएगी।
Work hard diligently, and you will achieve success.
Here, 'कस कर' describes the manner of holding or securing, implying tightness or firmness.