A compilation error occurs when a computer program's source code cannot be translated into machine code by a compiler. This can happen due to syntax mistakes, such as missing semicolons or incorrect variable names. The compiler checks the code for these errors before it can be executed, ensuring that the program is written correctly. When a compilation error is detected, the compiler provides error messages that help programmers identify and fix the issues. Common programming languages like Java, C++, and Python all have their own specific rules, and understanding these rules is essential for successful code compilation.