An Integrated Development Environment (IDE) is the application that you program in. IDEs let you edit source code and generally include tools that make it easier to code compared to using a plain text editor like Notepad. Lots of IDEs also allow you to download addons and themes, which make them very customizable to you and your needs.
There are many popular IDEs out there that you can download. Each one has their on advantages and disadvantages, so just try using a few and use the one you most prefer. Here are a list of popular IDE's that recommend.
Note: JetBrains IDEs such as IntelliJ and WebStorm do require a paid license for continued use; however, that license comes free with the [https://education.github.com/pack](GitHub Student Developer Pack).
List of Popular IDEs (In Alphabetical Order)