"The cat is light."
Translation:Die Katze ist leicht.
light is 'licht' in german.
Do you mean Licht with a capital L? That's the translation of "light" as a noun, as in "the sun gives light".
"light" as an adjective is leicht when it refers to weight and usually hell when it refers to brightness (e.g. "light blue" = *hellblau").
What is difference between das and die?
No difference in meaning between der - die - das. You simply have to memorise which word requires which article.
For example, don't remember Katze = "cat"; instead, remember die Katze = "the cat" so that you will know that Katze is a die word (and not a der word or a das word).