To know the reason why you need to override a function of parent theme in child theme, please refer to these sources:
https://paulund.co.uk/override-parent-theme-functions
http://code.tutsplus.com/tutorials/a-guide-to-overriding-parent-theme-functions-in-your-child-theme–cms-22623
To override a function, please add all codes of that function to functions.php file in childtheme:
