LLMs are happy to generate arbitrary strings. You might want it to spit out something along the lines of "Alice: 42" and then it spits out "hi, i'm helpful and

LLMs are happy to generate arbitrary strings. You might want it to spit out something along the lines of "Alice: 42" and then it spits out "hi, i'm helpful and Alice is exactly forty two, as far as I can tell, but I'm just a language model." So you give it a grammar that says the response has to be an uppercase letter followed by lowercase letters, then a colon, then a space, then digits, then it's done. Now, when it looks for that first token, it will only consider tokens that are compatible with that pattern. Then it'll continue with only next tokens that are compatible with the next parts of the pattern. These grammars do that with a flexible and useful kind of pattern.