This means that the teacher and learners focus on the target language only, often through an example at sentence level. This 'abstract' language contrasts with contextualised language.
Example
In young children, the ability to recognise and understand de-contextualised language is one milestone in the development of language.
In the classroom
Learners can produce some very interesting responses in activities that use de-contextualised language. For example, abstract terms can form a basis for creative writing, and many words games use de-contextualised vocabulary.
Further links:
https://www.teachingenglish.org.uk/article/revising-lexis-quality-or-quantity