Mobile First is a concept in web development that originated from Luke Wroblewski. It places importance on shifting the primary focus of development from desktop to mobile due to its pervasiveness in modern technology.
In Product Design, this concept places importance on the hierarchy of information and interaction. Because mobile is light and small, it warrants conciseness in communication. In turn, as designers, we are “forced” to prioritize important UI elements.
Even in its name, the Mobile First concept says that it is only a part of a whole. The second, third, and subsequent approaches could accommodate the items below the hierarchy.
In Visual Design, when there is no deliberate decision to design for a specific device, design for a mobile device—particularly a 320px-width, portrait orientation mobile device.