This is the 4084th most frequent Russian word.
одним
"Одним" means "one" or "single" (instrumental case of "один").
The word 'одним' is used here to indicate 'one' as an attribute of the noun 'днем', meaning 'day'.
Мы пошли в парк одним днем.
We went to the park one day.
The word 'одним' functions as an attribute for 'движением', indicating the singularity of the motion.
Он сделал всё одним движением.
He did everything in one motion.
In this sentence, 'одним' modifies 'махом', meaning 'stroke', to signify the completion in one single act.