This is the 12415th most frequent Russian word.
качай
"Качай" means "pump" or "swing" (imperative form).
In this sentence, 'качай' is used as an imperative form of the verb meaning 'download'.
Качай музыку только с безопасных сайтов.
Download music only from safe websites.
Here, 'качай' refers to the action of performing exercises to build muscle.
Качай пресс, чтобы укрепить мышцы живота.
Do sit-ups to strengthen your abdominal muscles.
In this context, 'качай' metaphorically means to influence or inspire change.