This means that the seller has completed the Seller's Property Disclosure forms. Disclosure topics include title, water rights, sewer, insulation, structure, fixtures, home owner associations, and any material defects or issues a buyer should know about. Sellers are required to disclose anything they have personal knowledge of, and should not speculate otherwise.
You typically get a copy of the disclosures after you make an offer, however, some parties will provide them in advance (usually with no issues of concern). Offers in Oregon on standard earnest money agreements typically provide for a 5 business day review of disclosures...usually with a non-conditional right to withdraw based on the findings.
The only parties exempt from filling out the 4-page Property Disclosures are government agencies, court-appointed receivers, banks or financial institutions (foreclosures), or builders of new construction.