HTML is the structure of the webpage containing all content and information. CSS is used to present this content and information in a way that is understandable and appealing to the viewer. Javascript is what controls the various elements on the webpage and is what makes a basic webpage a functioning website/webapp.
Control flow is a set of instructions that the computer is given and will complete when told to. A loop is these instructions repeated until the desired outcome is met. A real life situation would be if you were backing out of your drive way and a car pulls infront of you. You will press on your horn, the horn would sound, and you'll do this untill the other car move.
Accessing data using arrays is done by usings a numerical index to identify the specific data. Accessing data using objects is the same type of process except you don't need to know the numerical order of your data, you only have to know the property name.
The great thing about using functions in your JavaScript code is that you can code a set of instructions once into a function, then when you want the same function to happen again, all you have to do is call it instead of repeating the full set of instructions.