The document discusses collocations, which are predictable combinations of words that typically go together. It provides examples of different types of collocations like adverb + adjective. Collocations can be more or less fixed, with idioms being very fixed meanings and structures like "let's" being less fixed but still limited. Learning collocations is important because the brain stores language in chunks rather than individual words, so knowing collocations promotes fluency and efficiency in speaking, writing, listening and reading.