A syntax error occurs when the rules of a programming language are not followed, making the code impossible for the computer to understand. This can happen due to missing punctuation, incorrect use of keywords, or improper structure. For example, forgetting a closing bracket or using a variable without declaring it first can lead to syntax errors. When a syntax error is present, the program will not run, and the developer will receive an error message indicating the problem. Debugging tools and integrated development environments (IDEs) often highlight these errors, helping programmers quickly identify and fix them before executing the code.