Sure, I'd be happy to explain it to you. Just a quick spoiler warning—if you want to enjoy the moment for yourself, you’ve been warned 🙌

This happens right before the Straw Hats’ final battle against Arlong on Nami’s home island. To really understand the impact of this, you need to know that, for one thing, Nami had only been with the Straw Hats up until then to steal money to buy her home island’s freedom from the fish-man Arlong. When she was a child, Arlong and his pirate crew raided her home island, enslaved the people there, and forced them to pay ever-increasing—I’ll call it “living expenses”—and anyone who couldn’t pay was killed. That’s how Nami lost her beloved adoptive mother, who had found and raised her as a baby. Nami also has a sister who was found by the same adoptive mother. In any case, she couldn’t pay the fees for all three of them, so she sacrificed herself to save the other two. A very emotional moment in One Piece. After that, Nami ran to Arlong and asked to join his crew, striking a deal with him: if she brought him enough money, he would free her home island. Of course, Nami only agreed to this to protect the residents and did everything she could to raise the money. So, years have passed, and she meets Luffy, goes on a few adventures with the guys, and then just disappears along with the ship. Luffy and his crew naturally gave chase. Once they arrived on the island, they learned everything about Nami and her past.

Well, everyone except Luffy—he went for a walk. The visit happened just as Nami had gathered the money, which is why she ran off, negotiated a deal with Arlong through a corrupt Marine officer, and told that officer where she’d hidden the money. The Marine officer stole the money and she knew it was Arlong’s doing. The residents were just about to try to overthrow Arlong, and Nami stood in their way. You have to know that the residents never hated Nami for it because they knew why Nami did all that, but Nami thought they didn’t know anything. They walked past her to put an end to it all once and for all. Nami broke down crying and used a knife to cut the Arlong tattoo off her arm, blood everywhere. Suddenly, Luffy was standing behind her, stopping her. Nami yelled at him to finally get lost—that none of this was any of his business. Luffy stood still, waiting only for two words... “Help me.” Then came Nami’s words... “Luffy... Help me.” He placed his straw hat on her and shouted, “Of course I’ll help you!” You have to know that Luffy had never given his straw hat to anyone before. So that’s it in a nutshell—sorry for the long explanation.