A secondary or additional cardholder is someone you have requested to have access to your credit card account. Secondary cardholders get their own card, with their name on it, and when they use it the charges are made directly to your account.
But the legal responsibility for the credit card account always stays with the primary cardholder. So if your additional cardholder racks up a lot of debt on the card or goes over the credit limit, you will be the one dealing with the fallout as far as your issuer is concerned.
Before agreeing to add anyone as an additional cardholder on your credit card account, make sure you clarify how repayments will work and the responsibility you will both individually share for the account. Then, if anything does go wrong, call your issuer straight away and request that the additional cardholder be removed from your account.