ばれる means “to be found out” or “to be exposed,” especially when referring to a secret or lie. Its casual, vivid sound makes it more memorable than a formal expression.
In the song, the singer worries that a poorly constructed lie will quickly be discovered, while wishing that a more elaborate lie could remain hidden for 100 years. This word captures the tension between deception and truth that runs throughout the lyrics.
THE BLUE HEARTS ask a cheeky question here: "100億もの嘘を / ついたら今よりも / 立派になれるかな" [If I told ten billion lies, could I become more respectable than I am now?]. The singer then swaps "respectable" for "楽しくなれるかな" [could I have more fun?], as if lying were a shortcut to a better life. The word to learn is 嘘 (uso, a lie), and the title うそつき means "liar", someone who tells them (嘘をつく, "to tell a lie").
The plan has a catch. "下手な嘘ならすぐばれて / 寂しくなっちゃうよ" [a clumsy lie gets found out right away, and you end up lonely], so the answer is to go big: "せめて100年はばれない / たいした嘘をつく" [I'll tell a serious lie that won't be caught for at least 100 years]. That is funny and a little dark. Then the song turns: "嘘がホントになる / ホントが嘘になる" [lies become truth, truth becomes lies], and the singer wonders if anyone can tell the truth inside history. The ending piles up contradictions, "嘘は許さない 嘘は許してる" [I won't forgive lies, I forgive lies] and "嘘に傷ついて 嘘でほっとしてる" [hurt by lies, relieved by lies]. ホント is casual for 本当, "the truth", and ばれる is everyday slang for "to get found out".