A membrane keyboard is a keyboard where the keys are not separated, are covered by a transparent, soft plastic and have very little movement. Advantages of such a keyboard are portability as well as protection from damage and dirt. However, they are not widely popular because of inaccuracy and slow typing speed. Professionals generally prefer to use regular keyboards with their systems.
Techopedia Explains Membrane Keyboard
Membrane keyboards have pressure pads instead of a mechanical key which upon pressing, completes the circuit and sends the command of the given key. The keyboard has a flat surface printed with symbols, the pressing of which perform the same tasks as a regular keyboard with mechanical keys. Despite their portability and flexible design, membrane keyboards cannot be used for touch typing and can produce many errors upon quick typing. However, these keyboards are inexpensive to mass produce, and are more resistant against dirt and liquids than most other keyboards.