A divide-by-zero error happens when Python divides a number by zero.
Errors cause the program to crash. (This doesn't damage your computer at all. It's just that the computer doesn't know how to carry out this instruction, so it immediately stops the program by "crashing" rather than continue.)
An error that happens inside a try block will cause code in the except block to execute. That code can handle the error or display a message to the user so that the program can keep going.