The second amendment says:
A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.
The argument is whether the original framers meant this to refer to the existence of militias (which are regulated by government), or whether ordinary citizens can be included in it as well.
The idea is - what do the commas mean? And what did they mean in the 18th century?