This is the 11344th most frequent Polish word.
idącego
"Idącego" means "going" or "walking" (genitive or accusative singular masculine/neuter or genitive plural masculine personal form).
Here 'idącego' is a present participle used as an adjective to describe the man being seen.
Widzę idącego mężczyznę.
I see a man who is walking.
In this sentence, 'idącego' modifies 'kot', indicating the cat depicted in the illustration is walking.
Podoba mi się ilustracja idącego kota.
I like the illustration of a walking cat.
In this example, 'idącego' connects to 'człowieka', describing the man's activity at a prior time.