Machine code is a set of instruction which written in machine language (0 & 1 format) and it is directly executed by the CPU. Each instruction performs a specific type of task like loading data, initialize different type of task and also perform ALU operations.
In other way you can also say this, it is executable instruction running on computer system programs and different type of application. Machine code is also known as machine language that convert the low level source code to high level source code and only understood by the machine.
Example Machine Code