Same as every other programming language - a command that's incorrectly coded such as misspelling a keyword or specifying the wrong number or type of parameters to a function. Thes prevent a program from being compiled or interpreted and thus cannot be executed until fixed.
These are different from logic errors, which can be much more insidious and tougher to correct. For these, the program syntax is valid and the program will execute, but may abend or (even worse) run to completion but produce incorrect results. These are simply errors in the algorithm itself or in the way it was implemented.