An informational interview is an interview in which a job seeker is only looking for information, not for employment.
In an employment interview, the interviewer is trying to get a job. They have applied for a specific job and have been selected for an interview. If they do well in the interview, they might be offered the job. This means that an employment interview can be a very high-pressure situation.
By contrast, an informational interview is very low pressure. The job seeker is only looking to find out information about potential jobs. They are trying to find out more about a specific company. They might ask about the duties that go along with certain jobs, about the potential levels of pay, and about other things that might help them to decide if a given job or a given company would be ideal for them.
This sort of an interview is much less stressful than a regular employment interview. It is, however, still a valuable tool for a person who is trying to think about what sort of career they might pursue.