A typical embedded system consists of one or more microcontrollers and peripheral devices like memories, converters, I/O expanders, LCD drivers, sensors, matrix switches, etc. The complexity and the cost of connecting all those devices together must be kept to a minimum. The system must be designed in such a way that slower devices can communicate with the system without slowing down faster ones.
To satisfy these requirements a serial bus is needed. A bus means specification for the connections, protocol, formats, addresses and procedures that define the rules on the bus. This is exactly what I2C bus specifications define