Plugins are user-made addons which add to or change the behavior of KSP. They consist of one or more DLL files written in the C# programming language. Plugins allow changing how the game itself works, anything from showing new information windows to making a hovercraft part, or even new planets. Plugin support was added to KSP in version 0.14, and is available on all operating systems.
To use a Part Module Plugin, just drop the .dll file into KSP/GameData/ or a new sub folder inside it. To stop using a plugin, just delete it or move it outside the KSP/GameData/ folder.
Alternatively, if there is no specific .dll for the plugin, or if there is but any given installation instructions tell you otherwise, a good rule of thumb is to find the relevant folder within the plugin's GameData folder, and put that into KSP's GameData folder. If there are multiple folders at that level with names that approximate other plugins, it will generally do good to install them too as they are probably either dependencies of or enhancements for the plugin.
What it is generally not recommended to do is install the plugin's GameData folder (or folders on a similar level) into that of KSP, as this may confuse the root directory and some or all of the features of the mod will be depleted.
Plugins can be written in any text editor and compiled with a standalone compiler, but the easiest way to start coding is to use an Integrated Development Environment or IDE for short. An IDE is a program that helps you to write code (in our case the language C#). Listed here are some of the most popular options.
|MonoDevelop (aka Xamarin Studio)||LGPLv2 / MIT (Free)||Yes||Yes||Yes|
|JetBrains Rider||Commercial/Some open-source||Yes||Yes||Yes|
 Free without restrictions for individual users, larger teams have to pay to use multiple licences
See the below links for setting up your chosen IDE for building KSP plugins. Once you're ready to build, try creating your first module.