This is the 226th most frequent Polish word.
szybko
"Quickly" or "fast."
Here, 'szybko' functions as an adverb describing the verb 'biega', indicating the manner in which he runs.
On biega szybko.
He runs fast.
In this sentence, 'szybko' is modifying the verb 'działa' (works), showing the speed of the action.
Działa to bardzo szybko!
It works very quickly!
Here, 'szybko' qualifies the manner of learning, emphasizing the rapidity of acquiring the language.