responsive Accordion

Firstly I confused to write java script code for accordion because its my first time to wrote . selecting of node element and parent element is little bit confuse. it is an experience to select an element by tag name or class or id .

dear friends , could you help me how to select and what is node selectors and parent selector and overall dom methods ,target an element ,toggle an element. more importantly i understand the topic of writing code but selecting of elements by tag name, class name, class List, node selectors are more complex to me . any one help me to get out from this .