Open questions are questions which cannot be answered with just 'yes' or 'no'. Many of them begin with 'wh': why, where, who etc. Open questions can be compared to closed questions, which need only yes or no answers.

Example
'Why did you decide to study English?' is an example of an open question. Compare this with 'Did you decide to study English because it's useful for work?', a closed question.

In the classroom
Asking open questions can encourage learners to expand on their answers, which requires them to construct longer examples of language. A series of closed questions can be useful for checking concepts.

Further links:
https://www.teachingenglish.org.uk/article/assessment-learning-activities-0
https://www.teachingenglish.org.uk/article/test-question-types
https://www.teachingenglish.org.uk/article/perfect-teacher
https://www.teachingenglish.org.uk/article/fortune-teller