Reflection is the process of programmatically inspecting your code with other code, and digging into its internals at runtime. For example, you could inspect an assembly when it is loaded to see what classes and interfaces it implements so that you can call them. It is generally discouraged and should be avoided if possible. There are usually other ways to achieve the same result that don't require reflection, although it is occasionally useful.
Reflection is often bad for performance, and this is well-documented, but, as usual, it depends on what you're using it for. What is new is that there are significant changes to reflection for .NET Core. The API has changed and it is now optional. So, if you don't use reflection, you don't ...
