Bracket completion is a visual tool that the editor view uses to indicate you which bracket matches which. If you open any .tex file, and select any bracket, whether it be a parenthesis (), square brackets [] or braces {}, the editor will highlight the bracket and its match in yellow (this default color can be changed). So, for example, if you position the cursor on the braces in \section{Introduction}, you would see \section{Introduction} in the default yellow highlight, showing you the location of the beginning and ending brackets.