Machine code is the lowest level of programming language that a computer can understand directly. It consists of binary instructions, which are sequences of 0s and 1s, that tell the computer's hardware what operations to perform. Each type of computer architecture has its own specific machine code, making it unique to that system. When a programmer writes code in a higher-level language, such as Python or Java, it must be translated into machine code through a process called compilation or interpretation. This translation allows the computer to execute the instructions, enabling software applications to run effectively on the hardware.