A primary tradeline is an account that shows on your credit report as your account, versus being an authorized user’s account. It’s important to have a good mix of accounts on your credit report, including primary tradelines AND authorized user tradelines.
Ideally you want to have at LEAST 3 OPEN, POSITIVE accounts on your credit report, between primary tradelines and seasoned authorized user tradelines.
Primary tradelines go through a much more rigorous validation process than authorized user tradelines. There are several legitimate companies that exist for the sole purpose of reporting primary tradelines to your credit report.