"Lions are big cats."
Translation:Lwy to duże koty.
to is used with or instead of jest/są in noun is noun sentences.
Lwy to (są) duże koty.
Using "to" makes your life simpler because after "to" we use nominative case (the one you find in dictionary and use as subjects in the sentence)
If you don't use "to", you have to use instrumental case:
Lwy są dużymi kotami.