A financial restatement is a formal correction and re-issuance of previously filed financial statements, because the company has determined they contained a material error and should no longer be relied upon as originally reported. This is a genuinely different, more serious event than a routine, forward-looking accounting adjustment — a restatement specifically means something already reported and already relied upon by investors was materially wrong, not just a normal change going forward.
Companies are generally required to disclose the determination that a restatement is needed promptly, via a "non-reliance" 8-K (Item 4.02) — filed as soon as that determination is made, even before the actual corrected, restated financial statements are ready. This is exactly why a non-reliance 8-K is treated as one of the more serious 8-K trigger items: it's an admission, in real time, that the company's own previously-issued numbers can no longer be trusted as filed.
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