Suppose we discover this year that there was an error made last year that resulted in an expense of $20,000 not having been recorded. That means that last years profit should have been $20,000 less than what was reposted and that therefore the accumulated profits at the end of last year/start of this year should have been $20,000 lower.
When we discover the error we need to reduce the opening balance on accumulated profits/retained earnings by $20,000.
It is an adjust for the prior year. We only do this in respect of major errors.