It refers to joining the beginning of one word and the end of another to make a new word with a new meaning.
Example
Smog, from smoke and fog, and brunch, from breakfast and lunch, are examples of blends.
In the classroom
Discussion of blends can help learners understand how words are formed in English, e.g. Asking learners to identify root words. Learners can also invent new blend words.
Further links:
https://www.teachingenglish.org.uk/article/new-words-english