An IDE (Integrated Development Environment) is a tool that allows you to write code.
Visual Studio is one of the IDE's you can use to develop mods. While basic in its features, it will allow you to make content. You probably already installed a copy of Visual Studio while installing Unity. If not, you can install one from the Unity Hub by modifying your installation.
A correctly configured Visual Studio project will give you intelligent suggestions and syntax checking as you write mod C# files.
To use it, in the Unity editor, open Edit > Preferences..., go to the External Tools heading, select Visual Studio in the "External Script Editor" dropdown.
Unity can automatically generate a Visual Studio solution file for you to work in. If you have issues with that, you can click Regenerate project files(In External Tools menu). This will create a .sln file in the root folder of your Unity project.
JetBrains Rider is another IDE you can use. It is not free, however, students and some other people can get access to it for free. Rider features are much more advanced than Visual Studio, and it is recommended that if you can get access to it, you use it (IE you're a student).
Rider has smart auto-completions, code generation and a slew of suggestions on how to write your code better.
To use it, in the Unity editor, open Edit > Preferences..., go to the External Tools heading, select JetBrains Rider in the "External Script Editor" dropdown.