Rocks is a person or a group of people who were prominent 40 years ago, before the Pirate King Gol D. Roger came into power.
Not much is known about Rocks, especially among those who grew up after their time. They seem to have been powerful, given their prominence prior to Roger’s time and the fact that Garp could not imagine what the Marines would do if they came back into power. Garp is connected to them in some way, with Hina suggesting that he got the distinction of “Hero of the Marines” from dealing with them.
I put focus on the conversation between Garp and Hina. She says he’s the Hero because of something happened 40 years ago after killing the “Rocks” captain. I gave a look at Garp’s wikies and found out, he was chasing Shakky (Rayleigh’s wife) about 40 years ago. During the Sabaody Arc we get to know about it and Shakky said she just quit being a pirate 40 years ago to open her bar. The Marine may assume she’s actually dead and that Garp may killed her.
The odds are real that she was someone in the Rocks Pirates or Alliance. In support of this theory, recently Oda himself hinted that Shakky might be part of the Rocks Pirates. Here’s what he said, answering a fan’s question:
Reader: “Thinking about “pirate”, “40 years ago” and “Garp”, the one who comes to mind is Shakky! Do Shakky and Rox/Rocks/Locks have something to do with each other?”
Oda: “Huh…!! You remember things over 40 volumes ago!? N… no, they don’t!! Lalalala [TL/N: It’s the sound in japanese someone makes who got caught]”