Form S-1 is the document a company files to register shares for sale to the public in the United States. It is public on the SEC EDGAR database. It includes a description of the business, risk factors, how the money raised will be used, audited financial statements, management's discussion of results, executive pay and who owns the company.
It is often amended several times as the SEC reviews it and the details are settled, and the version circulated to investors early on is the preliminary prospectus. Many companies first submit a draft confidentially and make it public later, before the roadshow begins. Foreign private issuers generally use a different form, F-1.
An SEC review checks that required disclosure is present. It does not judge whether the investment is a good one.
When you read one, begin with the risk factors, the use of proceeds, the financial statements and the section on related-party transactions and ownership.