자 가다

let's go — said twice

-자 is the plain "let's", so 가자 is "let's go". Every line here repeats it — 가자 가자, or 가자 가자 가자 — which is simply how Koreans chivvy a group along; the repetition is the normal form, not extra emphasis. It is flat, informal speech, fine among friends or from an older person to a younger one, but not for a stranger or a senior. Polite equivalents are 갈까요? and 가시죠.

Register: very casual; friends and juniors only