This is the 6269th most frequent Polish word.
biegaj
"Run" (imperative form).
The word 'biegaj' is in the imperative form, instructing someone to run habitually or as practice.
Biegaj codziennie, aby poprawić swoją kondycję.
Run every day to improve your fitness.
Here, 'biegaj' is used in the negative imperative form, advising someone not to run quickly.
Nie biegaj tak szybko, bo możesz się zmęczyć.
Don't run so fast, or you might get tired.
In this sentence, 'biegają' is the third person plural present tense conjugation of 'biegaj', describing a general observation about running.