This isn't the best way to describe it. The following is an oversimplification.

Hiragana is a set of 46 characters, each representing either a vowel or a consonant-vowel combo. These characters are used to spell all native Japanese words. (There's a lot more than just this, but this is the most basic information.)

Katakana is a set of 46 characters, each representing either a vowel or consonant-vowel combo. These characters are used to spell foreign words (as best as possible.) Here's a Wikipedia article on English loanwords in Japanese.

These two writing systems are called, together, kana.

Then there's kanji, which are the symbols that mean an individual word or concept, and which are characters that were from China. 日 means "day" or "sun" and is pretty popular among beginners. (Or at least it's in all the beginner books.)

Combined, kana and kanji are the writing system.

The person asking if they need to know katakana is actually asking a normal question for foreigners. They just need to be reminded that in Japanese they can't expect the romanization of foreign words, because katakana is for people who speak Japanese to read foreign words. They don't all learn the English alphabet to read foreign loanwords!

I might know what Coca-Cola is, but unless I can read コカコーラ and understand that means koka-kola, I'm not going to know if it's being sold on a menu.

ETA; All words can be written in kana. Kanji isn't mandatory for written communication it's just that if you want to be proficient in the written language, you are required to know a certain number of kanji. I'd argue you need katakana more than kanji.