A more thorough explanation:

Legal defense is a method used by a defendant to oppose the plaintiff or prosecutor's case in a court of law. It is a defendant's stated reason why the plaintiff or prosecutor has no valid case. A legal defense can be affirmative, which means the defendant asserts facts and arguments that, if true, will defeat the plaintiff's or prosecution's claim, even if all the allegations in the complaint are true. Examples of affirmative defenses are duress (in a civil case) and insanity and self-defense (in a criminal case).

Another type of legal defense is a real defense, which is good against any possible claimant, so that the maker or drawer of a negotiable instrument can raise it even against a holder in due course. The ten real defenses are fraud in the factum, forgery of a necessary signature, adjudicated insanity that, under state law, renders the contract void from its inception, material alteration of the instrument, infancy, which renders the contract voidable under state law, illegality that renders the underlying contract void, duress, discharge in bankruptcy, or any discharge known to the holder in due course, a suretyship defense, and a statute of limitations.

For example, if a person is accused of murder, they may use the legal defense of self-defense, which is an affirmative defense. They would argue that they acted in self-defense and that their actions were justified. Another example is if a person is being sued for not paying a debt, they may use the legal defense of infancy, which is a real defense. They would argue that they were under the age of 18 when they signed the contract and, therefore, the contract is voidable under state law.