This is the 3514th most frequent Polish word.
świetlne
"Świetlne" means "light" (related to light, luminous).
In this sentence, 'świetlne' uses its adjective form to describe the noun 'zjawisko' (phenomenon), referring to a phenomenon involving light.
To było niesamowite zjawisko świetlne.
That was an amazing light phenomenon.
Here, 'świetlne' is in its adjective form 'świetlną', agreeing with 'prędkość' (speed) in number, case, and gender, to describe a property associated with light.
Prędkość światła w próżni jest stałą fizyczną, nazywaną prędkością świetlną.
The speed of light in a vacuum is a physical constant known as the light speed.
In this sentence, 'świetlną' is used to linguistically characterize the 'ścieżkę' (trail) as being composed of or related to light.