Definition
With reflection, C# code can look up a type from a string, list its fields and methods, read private values, and invoke methods it did not reference at compile time. Serializers, dependency injection, and some editor tools rely on it. Unity itself uses the names of serialized fields to load scene and prefab data.
Relevance
Reflection works because names stay in the assembly. Code can look up a private field or call a private method from a string, with no compile-time reference to that member. Private is a language rule for other source files. It is not a barrier at runtime.
Example
A debug console takes a string the player types, such as "Player.coins". It finds the type with Type.GetType, finds the field with GetField, and prints the live value. The console never referenced that field in code. The name in the string was enough.