Braces are a special tool that dentists or orthodontists put on teeth to make them straight. They are small metal or plastic pieces attached to the teeth with a thin wire connecting them. The braces slowly move the teeth into the right position over time. People usually wear braces for a few months or years until their teeth are straight. And just a small correction on your question: "Whether brace here means dental crown or tank-top ?" Instead we would say: In this context, do braces mean a dental crown or a tank top?

In this sentence, braces most likely refers to dental braces, which are used to straighten teeth. It would make sense to feel self-conscious about smiling due to having these visible on one's teeth, especially when first getting them. A "tank-top" might be referred to as a vest or sleeveless shirt in other English varieties, but this wouldn’t typically make someone self-conscious about smiling. Similarly, braces would not refer to dental crowns, which are caps placed on teeth, as these are typically less visible and wouldn’t cause self-consciousness about smiling.