A creditor that learns that a debt cannot be collected then charges off the debt. This means it is a loss for the creditor. In the case of bankruptcy, the debt cannot be sold to other parties for future collection effort.
A charge off simply means that your debtor wrote off the balance due as a bad debt before the debt was discharged in bankruptcy court. This will not really make too much of a difference on your credit report, as a bankruptcy is worse than a charge-off. Expect 10 years of impact on your credit report from this action.
If they charge off the debt, it means the loan originator says they cannot get it back and writes it off as a loss. You then will be taxed on that amount as income, because you spent the money and did not have to pay it off. This does hurt your credit score.