Machine code is a low-level programming language that consists of binary instructions directly understood by a computer's central processing unit (CPU). Each instruction corresponds to specific operations, such as arithmetic calculations or data movement, allowing the computer to perform tasks efficiently. Unlike high-level programming languages, which are more user-friendly and abstract, machine code is written in a format that is specific to each type of CPU. This means that machine code for one type of processor may not work on another, making it essential for software developers to consider the target hardware when writing programs.