So, a "trade-line" is basically how the credit industry refers to the accounts listed on your credit report. Whenever you open a credit card, take out a mortgage or finance a vehicle that requires an auto loan, this will add a new trade-in to your report.
People that have excellent credit have excellent trade-lines reflecting on their credit reports that show how well they pay their bills and use their credit wisely. On the flip side, people that have bad credit will have negative trade-lines on their credit report. These trade-lines might reflect several 30-day late payments or a credit card that is in default.
If you have bad credit, you can start to improve by focusing on repairing the existing trade-lines that you have, in addition to opening additional trade-lines that you consistently make on-time payments on. Ultimately, you want to continue to pay down your debt and pay your bills on time to improve your overall payment history.
And while I will never recommend "purchasing" trade-lines from a stranger, I absolutely recommend you being added as an authorized user on credit card by someone you KNOW and trust. (Family member or spouse) Ensure that this person has managed their account responsibly, has credit length of more than 9 years and they keep their utilization below 10%.