If you’ve ever looked in a dictionary, or at a Wikipedia entry, you’ll have seen IPA text, although you may not be familiar with what it is.
IPA is the International Phonetic Alphabet. It’s a set of phonemes, which are all the different sounds the mouth makes in language. It’s needed because our 26 letters (in English) don’t cut the whole range of sounds that we make. For example, the a in cat is pronounced differently from the a in bake, which is pronounced different still from the a in far.
So when you see a word being defined (as in a dictionary or on Wikipedia), it will often have these characters (such as /kəˈlʌmbɪdiː/), demonstrating how the word should be pronounced, which is really useful, assuming you know how to decode it.