Historically an example of good debt is real estate investment property. Ideally the property values go up overtime. (The tenant's rent pays the monthly mortgage while the landlord gets tax deductions for the interest on the loan and for any work/repairs he or she makes on the property.)
In other instances people went into debt to buy a fixer upper property and flip it to make a decent profit.
The vast majority of successful entrepreneurs use OPM (other people's money to build wealth). Very few of them climb to the top without borrowing money or going in debt.
Credit card debt, owing money on consumer products like clothing, jewelry, furniture, cars/accessories, TV & entertainment equipment are all examples of bad debt. These things don't increase in value and there are no tax deductions.
Good debt means you're in a position to sell, divest, or borrow against the principal. No one can borrow money against clothing.