This is the 2045th most frequent Russian word.
белая
"Белая" means "white" (feminine form) in English.
Here, "белая" describes the color of the cat, attributing it the property of being white.
Белая кошка сидит на крыше.
The white cat is sitting on the roof.
In this sentence, "белая" is in its accusative form because it describes a feminine noun in the direct object role.
Она надела белую рубашку на вечер.
She put on a white shirt for the evening.
"Белая" here modifies "бумага," indicating the color of the paper, in the nominative case for subject usage.