A person who attacks and robs ships at sea is known as a pirate. In the case of Luffy's definition, "being free is to be a pirate". He stops pirates from robbing and attacking innocent people, more like Pirate Hunter Zoro.
First of all, the context is important in trying to respond to your question.
Luffy declares himself as a pirate and even going as far as saying he will be the Pirate King.
His extended family is closely related to piracy and other questionable lifestyles, like
his father is a Revolutionary - opposing the World Government - and his brother was the son of the former Pirate King.
On his journey, he came across representatives of the World Government (ships convoys, delegates, Admirals, etc). The first island he disembarks on is a WG military outpost where he damages the building and defeats the outpost commander - therefore receiving his first bounty from the WG and marking the start of his fame as an outlaw. He even attacks and destroys ships at sea on some occasions.
As @JaysheelUtekar pointed out, sailing with a black flag will brand you as a pirate whatever your actions may be, fact stated by the WG in the beginning of the story.
On a side note, the WG considers a pirate almost everyone that is at sea and produces damages to and/or opposes the WG in any way.
Luffy's desire to become a pirate is symbolic of his idea of "Freedom". Tales from Red Haired Shanks fuelled this idea to the point where becoming the "King of the Pirates" would allow his to achieve ultimate freedom.
Although he hasn't achieved the status as king yet, he has had a taste of true freedom. After Bonney questions why Luffy has white hair in his Yonko bounty (5th Gear) his response is "Oh, That's What I Look Like When I'm Free!!".
Other pirates in One-Piece are typically aggressive and would willy-nilly murder and pillage as stereotypical pirates. Luffy's ideology however contradicts that, which does make him out to act non-pirate like but that's to do with how other pirates are contrasted.
My interpretation of wording:
Luffy wants to live free. To Luffy, the Pirate King is "Free-er than anyone," and that is what Luffy wants to be.
Luffy also wants to unite the world against all those that use power to oppress and control. Luffy does want to steal this power and control from the secret societies and one world government.... only to give it back to the people.