A more thorough explanation:

The Principal Register is a list of trademarks that meet certain federal filing standards. To be placed on the Principal Register, the trademark owner must pay a fee and file an application with the U.S. Patent and Trademark Office. The application must include information such as the applicant's address and citizenship, the date of first use of the mark, the goods associated with the mark, and a drawing of the mark.

In addition to meeting these requirements, the applicant must also show that they are the rightful owner of the mark, that the mark is used in commerce, and that no one else has the right to use a similar mark that could cause confusion or deception.

Benefits of having a mark on the Principal Register include protection against copying, the ability to sue to stop copying, and immunity from legal challenge after five years of continuous use.

For example, if a company creates a new logo and wants to protect it from being copied by competitors, they can apply to have it placed on the Principal Register. Once the mark is registered, the company can take legal action against anyone who tries to use a similar logo in a way that could cause confusion or deception.

Some marks that do not meet the requirements for the Principal Register may still be eligible for the Supplemental Register.