You need to be prepared with this question because the way you answer the question decides whether the interviewer will listen to you throughout the rest of the interview or switch to their phones and pretend to listen to the interview :p
The worst part is that it's not as simple as it looks. The interviewer DON'T WANT TO KNOW ABOUT YOU. They only want to know about one thing, i.e.
Are you fit for the job or not?
So the answer to this question can be broken down into 3 parts:
1. Who you are? The first line should summarize your experience or your education plus some relevant adjectives that fit your personality.
2. What are some of your accomplishments? Try showcasing some of your best and unique accomplishments, don't try to use superlatives here because it makes you sound arrogant. So the following line should consist of a list of achievements and also some proof of performance (adding numbers to the sentence always makes it sound more credible). Please try to relate them to the job that you're applying for.
3. Are you fit for the role? You need to end your answer with why you're applying for the job and how you're fit for the job, read the JD carefully, and mention how your skills align with the job.
Please note that you have a maximum of 1 min to 90 sec to complete this answer, don't bore the interviewer with your entire life story XD
Just to add, don't sound too mechanical and be confident while answering this or any such HR-related question.