Reformulation is an error correction technique. In reformulation, the teacher repeats what the learner has said but correctly, without drawing attention explicitly to the error itself. This technique offers a correct example of relevant language to a learner, at a time when the learner may be ready to notice the difference between what they say, and the correct version.

Example
The learner says 'I have been to the swimming pool last week'. The teacher replies 'You went to the swimming pool last week? So did I'.

In the classroom
Reformulation is one of many ways to correct learners' spoken errors. Others include echo correction, which is repeating the error with emphasis, and using facial expressions or gesture. These correction techniques avoid giving the right answer and encourage the learner to correct themselves.

See also:
https://www.teachingenglish.org.uk/article/delayed-correction
https://www.teachingenglish.org.uk/article/spot-correction

Further links:
https://www.teachingenglish.org.uk/article/error-correction-2
https://www.teachingenglish.org.uk/article/teacher-talk-error-correction
https://www.teachingenglish.org.uk/article/lexical-approach-2-what-does-lexical-approach-look
https://www.teachingenglish.org.uk/article/delexicalised-verbs-2