Display questions are questions you ask to see if the person you are speaking to knows the answer. In an ELT classroom, this normally means questions teachers ask learners to see if they understand or remember something. Display questions can be compared to referential questions, which are questions you ask because you don't know the answer.
The teacher asks a learner ‘What is the past simple form of leave?'
In the classroom
Display questions clearly lack the communicative quality and authenticity of referential questions, but they are an important tool in the classroom, not only for the teacher to be able to check and test their learners, but also as a source of listening practice. One of the first things a beginner learns in English is how to understand and answer display questions.