This is the 12104th most frequent Polish word.
ludzki
"Human" or "humane."
In this sentence, 'ludzkie' modifies 'zachowanie' (behavior), used to describe it as typical of or characteristic of humans.
To zachowanie było bardzo ludzkie.
That behavior was very human.
The term 'ludzka' describes 'natura' (nature), referring to complexities tied to humans in general.
Ludzka natura jest skomplikowana.
Human nature is complex.
The word 'ludzki' here is used as an adjective to describe the noun 'rozum' (mind), indicating its association with humans.