Bracket completion is a visual tool that the editor view uses to alert to user to brackets that match each other. If you open any .tex extension file and select any bracket, whether it be a (), [] or {} the bracket selected will highlight its matching bracket in yellow (default color, but can be changed). So, an example would be if a user clicked the open bracket in the example \section{Introduction}, they would see \section{Introduction} in the default yellow highlight telling them the location of the beginning and ending brackets.