Implicit requirements are those features of a product or app which are so fundamental that they don’t need to be explicitly specified during development.
Like flicking a light-switch and seeing the bulb illuminate, or pressing the accelerator and feeling the car move forward, some things in life are just expected.
When it comes to product development, the same concept applies. Implicit requirements are effectively the core mechanics of a product; elements or characteristics both developers and the end-user will expect as a bare minimum.
Implicit requirements are rarely documented on the development roadmap — they are, instead, universally understood as must-haves.