It means you have to go to court to prove you did the legal exemption to the law

An example in Texas I know offhand is on our age of consent law

It’s 17.

So if a 17+ sleeps with someone under 17 they can be charged with statutory rape

In Texas it is an affirmative defense if they’re within 3 years of each other.

So a 18 and 16 year old are legally okay in theory. (Sometimes it’s still prosecuted but it’s rarer)

Affirmative defenses are there usually as a check and balance thing. An over zealous prosecutor might charge someone but a judge and defense team has an out to throw out the case

Another way to look at it is “a reasonable person wouldn’t see this as a big deal”

Like self defense or a high school senior and sophomore sleeping with each other