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 semicolon at the end of a statement in Java can lead to a syntax error. When a syntax error is present, the program will not run, and the developer will receive an error message indicating the problem. Identifying and fixing these errors is crucial for successful coding, as they prevent the program from executing as intended.