It is probably 'Which is' It refers to something before it in a sentence. E.g. I can see a car, which is black.

I think it should read "which is" and it has the same meaning as "that is" (che è). As dougiebearing said in their answer, it refers to the previous noun in a sentence and is followed by an adjective to describe the noun with. Cosi, la frase "I can see a car, which is black" è si dice come "Riesco a vedere una macchina, che è nero." in Italiano.