The default text editor on Windows - Notepad - is very basic. You can edit raw text with it, but that's all. If you need to open multiple files, edit files that use Linux-style line returns, or edit code, you will quickly feel the need of a more advanced editor.
Notepad++ is a lightweight program that aims to replace Notepad. It supports syntax highlighting of a great number of programming languages. When you edit code, special keywords are colored differently than regular words which makes it easier to understand the structure of a program. Furthermore, as you type code Notepad++ displays suggestions so that you can program faster and with less errors.
One of the great features of this text editor is its ability to save your files in real-time to its cache. This means that you can close the editor and open it later without losing your changes, even if you didn't save the files. This also means that you won't lose unsaved changes if your computer crashes.
While Notepad++ is a great code editor, it's not a full IDE as compiling code and executing it is not straightforward, some configuration is needed. If you're not comfortable with build systems, it will not prove easy.
Another downside of Notepad++ is its lack of modern features. While it is regularly updated, no major features have been added for a few years. For instance, the git versioning system isn't integrated, you will need to use a plugin to enable it.
To sum it all up, Notepad++ is a lightweight and fast code editor that is suited for an occasional use. If you are willing to easily build your project and execute it, you should probably use a full interactive development environment program.