Another user has locked a row that this statement selects. If you executed the statement interactively, you can do one of two things. You can wait a short time and reenter the statement. Or you can execute the statement SET LOCK MODE TO WAIT, after which you will rarely see this message again.
In a program, you should roll back the current transaction, wait for an interval of random length, and rerun the transaction. If you run the transaction with a LOCK MODE of WAIT, you can reduce the frequency of this error. But it can still arise in some cases, such as when deadlock is detected. An ISAM error code (-107, -113, -134, -143, -144, or -154) usually accompanies this error, and a program might need to make a different response to each of these errors.