This is the 13858th most frequent Polish word.
polnej
"Polnej" means "of a field" or "field's" in English, typically as a feminine genitive adjective.
Here, 'polnej' modifies 'rzeki' (river), indicating that the river is in a meadow or field.
Spacerowałem brzegiem polnej rzeki.
I was walking along the edge of the meadow river.
In this context, 'polnej drogi' (country road) refers to a road typical of rural, field landscapes, enhancing descriptive details.
Namalowałem obraz polnej drogi o zachodzie słońca.
I painted a picture of a country road at sunset.
The phrase 'polnej łące' (meadow field) uses 'polnej' to specify that the meadow is situated in a countryside or field-like area.