What does mean that

"all political powers comes from the barrel of a gun"

There is soft power and hard power in geopolitics.

Soft power is non militant and usually characterized by incentives. 
Hard power is sometimes militant and characterized by direct intervention.

For instance, when the United States of America wanted to prevent Iran from gaining nuclear weapons, they enacted a deal that incentivised Iran to CHOOSE not to explore nuclear weapons. The main incentive was lifting economic sanctions that prevented Iran from trading with other countries. 

If USA had used hard power, they would have bombed nuclear refining facilities.

Here's the catch, would the USA have the ability to use the easier, more peaceful, soft power if it wasn't as strong militarily? No, because the US Navy's ability to enforce maritime law, protect trade routes over oceans, etc give the USA political power. Using soft power may be a wise idea, but it is impossible without the strength to enforce, IE military power.