A more thorough explanation:

An expert witness is someone who has a lot of knowledge and experience in a specific field. They are called to give their professional opinion in court or to a tribunal about a particular matter that is in dispute. There are two types of witnesses in the US legal system: lay witnesses and expert witnesses. A lay witness can only talk about what they saw or heard, but an expert witness can give their opinion based on their expertise.

For example, a digital forensics expert can help in a court case by analyzing deleted computer files, consulting on how to preserve or retrieve data, and determining how a computer was used. They can explain digital subject matter so that non-experts can understand. In a shooting case, people who saw or heard the shooting may have relevant information to testify how many shots were fired, but only an expert who has qualifications would be allowed to answer questions about the path of the bullet.

Before trial, all experts must prepare a report summarizing their analysis and conclusions, and share the report with all other parties. This allows other parties to effectively cross-examine the expert. Expert testimony is not limited to matters beyond the understanding of the ordinary juror. Instead, experts may testify on any subject within their area of expertise so long as their testimony will assist the jury.

Overall, an expert witness is an important part of the legal system because they can provide specialized knowledge and help the court make informed decisions.