That basically means "Master Swordsman"
In Ryuma's One Shot, there were Two Villains, one was called "First Rate Swordsman" & other was called "Second Rate Swordsman"
But it doesn't mean all Master Swordsmen are comparable
Cuz again, in Ryuma's One Shot, even though the Main Villain was a Master Swordsman, Ryuma still One-Shot him (Not even Prime Ryuma cuz he said he still needs to get Stronger & since it's Canon now, he didn't have Black Sword & wasn't One-Eyed back then)
So Master Swordsman are those who reached a certain Very High Level in Swordsmanship, but they can still be bested by other Master Swordsmen in various difficulties
Imo Oden, Vista, Shanks, Rayleigh ... etc are also Master Swordsmen
Zoro needs to Surpass them All, and since he is Main Character, then his Dream is about all Times, not just Current Swordsmen