Полигон literally translates to "proving ground" or "polygon". It's an interesting word choice that immediately brings to mind a place for testing or a military training area.
In the context of the song, Jony uses "полигон" metaphorically to describe a dangerous or challenging emotional space, perhaps where relationships are tested or where one is constantly under attack from Cupid's arrows. This unique imagery makes the word stand out and adds a layer of intrigue to the lyrics.
A полигон is a firing range, and Jony has decided that's what love looks like from where he's standing. Cupid's arrows are flying, but he doesn't see them and doesn't hear them, because he's so far away, and honestly he doesn't even feel like coming closer. He's heard enough stories already. The pow-pow-pow running through the chorus is the gunfire, and it's also just an extremely good hook.
Then the guard slips. He jumps up onto the roof where you can see everything, catches sight of some lace, and can barely breathe. The sun is beating down, her hand is leading him, the sea will cool them off, and he says outright that she pulls at him like a magnet. Somewhere in there he admits an arrow will get him one day. For now he's promising himself, and her, that he'll find her and rewrite the story.