This is the 4423rd most frequent Polish word.
jadący
"Jadący" means "riding" or "driving" in English.
In this sentence, 'jadący' acts as an adjective describing the car (samochód), indicating that the car is currently moving.
Samochód jadący po ulicy nagle się zatrzymał.
The car driving on the street suddenly stopped.
Here, 'jadący' is used as an adjective to describe the boy (chłopiec), specifying that he is in the act of riding a bicycle.
Jadący na rowerze chłopiec miał na sobie kask.
The boy riding a bicycle was wearing a helmet.
In this usage, 'jadącego' modifies 'tramwaj' in the accusative case, showing the ongoing action of the tram moving as perceived.