If a window pops up in MYOB and says;

 

“One or more parts of this journal entry have been reconciled.  Changing or deleting this entry will affect your next reconciliation.”

It means the entry that you are looking at has been matched or reconciled typically to a bank or credit card statement.  It is perfectly acceptable for you to look at the transaction; however, if you delete, or alter the TOTAL monetary values, it will alter the reconciled accounts.  Sometimes, when correcting transactions this needs to be done, but a correction reconciliation, for the period effected will need to be done, if these changes are made.  Why?  Because if the balance brought forward is $100, and a transaction of $50 is deleted, the information in MYOB will now be different to the bank statement.  A correction reconciliation will be needed to align the balances brought forward.