In simple words, it is a process for creating a game. The developer has to do the entire job related to the ideas, visual design, and the coding of the game.
Important aspects of a game that a developer must design and develop include:
Objectives
Setting and level design
Player (Character) design
Mechanics and movement
Challenges: Objects and enemies
Scoring
Sound
User Interface
As a game developer you will need to understand, develop, and master all these aspects to make a successful game!