They can be words that always go together, such as fixed collocations, or that commonly do, such as certain grammatical structures that follow rules. A listener or reader uses their knowledge of chunks to help them predict meaning and therefore be able to process language in real time. Chunks include lexical phrases, set phrases, and fixed phrases.
Example
'Utter disaster', 'by the way', 'at the end of the day', 'encourage + someone + infinitive', 'dependent + on' are all examples of chunks.
In the classroom
Areas of work such as idioms, collocations and verb patterns all focus on types of chunks. Learners can be encouraged to identify and record lexical and grammatical chunks as they find them.
Further links:
https://www.teachingenglish.org.uk/article/lexical-approach-1-what-does-lexical-approach-look
https://www.teachingenglish.org.uk/blogs/shelliscfc/teaching-chunks-very-young-learners