The units are words, chunks formed by collocations, and fixed phrases.
Example
The phrase 'Rescue attempts are being hampered by bad weather' is a chunk of language, and almost a fixed phrase. It is formed by the collocations 'Rescue' + 'attempt', 'rescue attempt' + 'hampered', 'hampered' + 'by', 'hampered by' + 'bad weather'.
In the classroom
A simple activity to incorporate the lexical approach is to encourage learners to identify and record lexical chunks and fixed phrases in texts they read.
Further links:
https://www.teachingenglish.org.uk/article/lexical-approach-1-what-does-lexical-approach-look
https://www.teachingenglish.org.uk/article/lexical-approach-2-what-does-lexical-approach-look
https://www.teachingenglish.org.uk/article/lexical-approach-classroom-activities