This is the 2422nd most frequent Polish word.
żyją
"They live."
The word 'żyją' (live) indicates the existence or manner of living of people on Earth in this context. It is in the third person plural present tense form of the verb 'żyć'.
Ludzie na ziemi żyją w harmonii.
People on Earth live in harmony.
Here 'żyją' (live) is used to express the mode of existence of animals in their natural habitat. It is the present tense plural form of 'żyć'.
Zwierzęta w lasach żyją bezpiecznie.
Animals in forests live safely.
The verb 'żyją' (live) metaphorically represents the source of livelihood or subsistence in this sentence.