These sub-skills are specific behaviours that language users do in order to be effective in each of the skills.
Example
Learners developing the skill of listening need to have the sub-skill of being able to recognise contracted forms in connected speech.
In the classroom
Learners spend time developing a wide range of sub-skills as they build each of the four skills. Amongst the sub-skills focussed on are scanning and skimming in reading, organisational and editing skills in writing, recognition of connected speech and understanding gist in listening, and pronunciation and intonation in speaking.
Further links:
https://www.teachingenglish.org.uk/teaching-reading-writing
https://www.teachingenglish.org.uk/teacher-development/teaching-speaking
https://www.teachingenglish.org.uk/article/teens-listening-skills
https://www.teachingenglish.org.uk/article/rachael-harris-bloomifying-stories-inclusive-practice-classroom