Retrieval & search
Finding texts that contain the same words as the question, ranked by how often those words appear and how rare they are overall.
Formal
A way of searching that scores each stored text by the words it shares with the question, giving more weight to words that are rare across all texts while each extra repeat of a word in one text adds less than the one before; BM25 is the most common scoring recipe.
In plain English
Like the index at the back of a book - look up "tax" and it lists every page where that exact word is printed, but not the page that says "duty".
In practice
An IT support worker at a region pastes error code 4031 from the patient record system into the IT knowledge base, and keyword search returns the one article that mentions that exact code.
Why it matters
It is fast, cheap and easy to explain, and it wins on names, product numbers and codes, where meaning-based search tends to treat near matches as good enough.