It's likely about the adventures in the Sea of Thieves world. It might feature pirates, their quests for treasure, battles on the high seas, and the various strange and dangerous creatures that inhabit that world.
I'm not sure specifically what the 'curse by the sea full story' is without more context. It could be about a legend where a curse is associated with the sea, perhaps something like sailors being cursed with bad luck on the ocean due to an ancient wrong or an encounter with a malevolent sea creature.
Yes, in a sense. Other players might take credit for things you've done in the game, like finding a valuable treasure or completing a difficult quest first. They could brag about it as if it were their own accomplishment, which is like stealing your story.