In the default case, whenever an error is detected by IDL during the execution of a program, program execution stops and an error message is printed. The execution context is that of the program unit (procedure, function, or main program) in which the error occurred.
Sometimes it is possible to recover from an error by manually entering statements to correct the problem. Possibilities include setting the values of variables, closing files, etc., and then entering the command .CONTINUE, which resumes execution of the program unit at the beginning of the statement that caused the error.
As an example, if an error occurs because an undefined variable is referenced, you can simply define the variable from the keyboard, then continue execution with .CON. Of course, this is a temporary solution. You should still edit the program file to fix the problem permanently.