A more thorough explanation:

Definition: In chief means principal or main, as opposed to collateral or incidental. It is often used to denote the part of a trial in which the main body of evidence is presented.

Example 1: The CEO is the in chief of the company, responsible for making all major decisions.

Example 2: During the case-in-chief, the prosecution presents their main evidence against the defendant.

The examples illustrate that "in chief" refers to the most important or primary aspect of something. In example 1, the CEO is the most important person in the company, while in example 2, the case-in-chief is the most important part of a trial where the main evidence is presented.