Anything that's secondhand has been used before. Your secondhand boots may look brand new, but they were owned by someone else before you bought them.
Things that aren't new are secondhand, so you might buy a secondhand car from your grandma or shop at a secondhand clothing store to get good deals. You can also use this adjective to mean "not from an original or primary source." If your sister tells you that her best friend told her that he saw a famous movie star at the coffee shop, that's secondhand information.