This is the 9245th most frequent Russian word.
солнечной
"Sunny" or "of the sun."
The word 'солнечной' is used here as a descriptive adjective in the instrumental case, describing the weather's characteristics as sunny.
Сегодня погода солнечной и теплой.
Today's weather is sunny and warm.
In this sentence, 'солнечной' is an adjective in the prepositional case, modifying 'стороне' (side) to specify it as sunny.
Мы отдыхали на солнечной стороне пляжа.
We rested on the sunny side of the beach.
Here, 'солнечной' is an adjective modifying 'польностью' (completely) to describe the entirety of the field as being filled with sunlight.