Voluntary contribution i.e. noncoercedly paying dues if you find it in your interests to do so.
Ok so how do you pay the army for example? You hire soldiers and you tell them "ok everyone, you might get paid if we have enough donations. If we don't well you'll have to eat your hands but we might get enough don't worry!"
That is what militias are for.
"A well regulated Militia, being necessary to the security of a free State..."
A militia is a group of people volunteering for being the police no? You want this all around your country you sure? xD