A semi-structured interview is a qualitative method of inquiry that combines a pre-determined set of open questions (questions that prompt discussion) with the opportunity for the interviewer to explore particular themes or responses further.
A semi-structured interview does not limit respondents to a set of pre-determined answers (unlike a structured questionnaire).
Semi-structured interviews are used to understand how interventions work and how they could be improved. It also allows respondents to discuss and raise issues that you may not have considered.