An invisible interface allows the user to focus on performing a task, and not on how it should be performed. An invisible interface has a low or non-existent learning curve. Good invisible interfaces often contain successful parallels with our physical surroundings, which dictate certain limitations. If this is not taken into consideration, the result is often confusing. The user shouldn't have to think, which means that the optimal interface is simple.
Simplicity is obtained by eliminating unnecessary details and explanations.
To me, simplicity is providing the right options at the right time, no more, no less. Unfortunately, the right time differs from person to person. Simplicity is the hardest thing in the world to achieve. It's a good idea to implement some sort of help system when creating digital interfaces. The first web pages consisted of static texts that sometimes featured simple images.