You have used some arrays already. As mentioned before it is hard to explain any one part of Web development without bringing up others so you needed to be able to show the content of arrays without really knowing what they are. Now though you can learn properly about these things.
You will also learn about loops and how useful they are to go through rows of data in an array. Loops are used in almost all programming languages. They allow you to write some code and then have it run many times. One example might be to show a list of ten products on a screen. You want each one to look much the same (product number, image of product, description) so you set up a loop which uses echo to create those HTML elements ten times but with a different product each time. If you get into programming any graphic applications you will also find many other uses for loops. It is fair to say that loops are what make computers useful.