Meta data that is stored in the assembly to aid reflection devs when they are viewed through reflection or the IDE. The aim is to have the component "self describing" itself.
Example is
Obsolete: will give off compile time warnings, or draganddrop attribute on a win forms control.
They can have both Positional parameters and Named parameters - positional go first - e.g.
DllImport("User32.dll", EntryPoint="MessageBox")The url of documentation for a method can be an attribute.