Well, you may prepare yourself by reading the job ads, and have an understanding of the roles and responsibility written. However, most write-ups are simple advertisements, and you know advertisements are there to capture your attention, very vague and little facts. So, then you have an understanding that this company is selling you a job.
There is one source where you may refer to, to have a better understanding of the job itself, is through O*NET. Here you can get more information about the job, including:
- Tasks to perform the job
- Tools and technology used
- Knowledge, skills and abilities required
- Detailed work activities
- Level of education required
- even the Work context and etc.
So, that is the tricky part of answering this question. It is not about the level of your understanding towards the job, but to express your humble understanding and let the interviewer guide you further of what they are offering, and what they expect from you.
You may briefly express your understanding of this vacancy and emphasize that is what you understand from the job posting, show your curiosity towards the job by inviting the interviewer to guide you further.
"bla bla bla is my understanding of the job posting, correct me if I am wrong. Perhaps, Mr/Ms X may give me a job preview, so that I would be able to further understand?"