"Balance" is a front end token that extends the current selection to include the next matching pair of parentheses, brackets, or curly brackets.
This token is equivalent to the menu command Edit Check Balance.
Each time you use "Balance", Mathematica expands the selection in both directions until it finds pairs of matching parentheses, brackets, or braces. Repeating the command expands the selection one level at a time.
To execute a front end token, you typically evaluate a command of the form FrontEndExecute[FrontEndToken[token]].