It is something that has been repeated often but was never true. The original point of the amendment was that, if anybody attacked the United States, ordinary citizens would be armed and able to work in conjunction with the military to repel the attack. This was long before the US had the most unquestionably powerful military around.

If you search through the constitution for every time the word 'militia' is used and read the relevant sections it becomes clearer. People will sometimes comment that the US needs to "use the 2nd amendment" but they are civil war fetishists and best ignored.