"It is a cat."
A nice exemple where the "A" in 『AはBです』is explicitely stated.
(with である being a litterary version of です, and 吾輩(わがはい, also written 我輩) is a an old first person pronoun);
吾輩は猫である, I am a cat, is a novel from 夏目漱石(なつめ・そうせき). As more than 100 years have passed since the author death, the whole text is now public domain, and you can find it complete (translitered to modern orthography and with some furigana) here.
です is a special verb; it functions as the Japanese copula which is the equivalent of "is/am/are". If you look at sentences like a mathematical formula, the copula is basically an equals sign. For example "it is a cat" would be "it = a cat", or "I am John" would be "I = John". です is a similar sort of equals sign, in Japanese.
At this point, you're probably thinking: hang on a minute, you need two things for an equation but the Japanese sentence only has one, ねこ. To which I'd say calm down Charlie Brown, I'm getting there.
Because of Japanese word order rules, the verb (in this case です) always needs to go last, so we can't organize our sentence exactly like a mathematical equation, such as A = B (unlike "A is B" in English). This is the role は or が plays in these simple sentences where です is the copula*: it tells you which word is A, and by default, whatever is next to です is B. The general structure for "A is B" in Japanese is 「AはBです」or 「AがBです」(depends on emphasis/context).
However, in Japanese, if "A" is already obvious from previous context, it's common practice to leave it out. For example, if someone asks you "what's your name", you can just say "John" and it's obvious you mean "my name is John". And because you've left "A" out, there's no need to mark it with は or が, and you're just left with Bです. If you wanted to as は or が, you would also need to at the "A" that it's pointing to.
When we translate ねこです, we don't know what "A" is because we have no context, so we just use the English generic pronoun "it" instead.
*There are grammar structures in Japanese where です is not being used as a copula, but they're rather a bit more advanced.