Descriptive grammar is a set of rules about language based on how it is actually used. In descriptive grammar there is no right or wrong language. It can be contrasted with prescriptive grammar, which is a set of rules based on how people, mostly writers of style books and grammar text books, think language should be used. See https://www.teachingenglish.org.uk/article/descriptive-grammar .
Descriptive grammar provides rules based on what is acceptable in different contexts, reflecting how language changes and how language is actually used. It can be contrasted with prescriptive grammar, which prescribes rules based on how writers of style books and grammar text books think language should be used. See https://www.teachingenglish.org.uk/article/descriptive-grammar or https://linguapress.com/grammar/descriptive-grammar-english.htm#definition
