Blindagem literally translates to "armor" or "shielding". It's a strong and evocative word, often used in contexts of protection, like bulletproof vests or armored vehicles.
In this song, the singer uses it metaphorically to describe the emotional barrier or "armor" that the love interest has built around themselves due to past hurt. The line "'Tá criando entre a gente uma blindagem" means "It's creating a shield between us," highlighting the emotional distance caused by insecurity.
Wesley Safadão has exactly one job here: convince a woman that he is not her ex. He does not pretend the ex was harmless. He concedes it in plain words, "Entendo ele te fez sofrer" ("I understand he made you suffer"), and then declines to serve the sentence for it. His real complaint is the wall that went up afterwards. Her caution is "criando entre a gente uma blindagem" ("creating a shield between us"), and a shield built out of someone else's behaviour keeps out the wrong man.
The title is the whole argument in five words. "Ele é ele eu sou eu" ("he's him, I'm me") lands three times in each chorus and then breaks into its halves, "Ele é ele" ("he's him") and "Eu sou eu" ("I'm me"), as though saying it slower will finally make it stick. He asks for one thing, "Se deixar eu te amar" ("if you let me love you"), and puts up one promise against the ex's record: "Não vou dizer Adeus" ("I won't say goodbye"). Os Barões da Pisadinha set all of this on a bouncing pisadinha groove, which is a strange place to park a man pleading his case, and it works.