Reflection is the ability of types to provide information about themselves. For example, an assembly can tell you what it contains, a type can tell you its methods, properties and so on.
Dynamically providing this information is useful in many ways. One simple example to think about is metadata used by web services - when a person "consumes" a web service, a proxy class is generated for their client. This proxy is generated from a WSDL document and that most often is generated from type metadata generated via reflection.
Another simple example is dynamically loading types in order to perform some unit of work. One project I worked on utilized reflection to load "rules" from a database to apply to inputs in the system.
