This is the 15244th most frequent Polish word.
siedzącym
"Siedzącym" means "sitting" (referring to someone who is seated).
In this sentence, 'siedzącym' is used as an adjective describing the boy ('chłopcu'), indicating his action of sitting.
Przy stole siedzącym chłopcu podano talerz zupy.
A bowl of soup was given to the boy sitting at the table.
Here, 'siedzącym' is employed to describe the person ('osobą') and denotes the individual's posture or state of sitting.
Rozmawiałem z osobą siedzącym na ławce w parku.
I spoke with the person (male) sitting on the bench in the park.
In this case, 'siedzącym' qualifies the noun 'kolegą', which refers to the seated friend.