What is an IPO, and what did it mean when Anthropic filed an S-1 on Monday?

An IPO, or initial public offering, is the first time a private company sells its shares to the public so they can trade on a stock exchange. Before that, the shares are mostly held by founders, employees and private investors. In the U.S., a company has to file a registration statement with the SEC first, usually on a form called the S-1. Inside it is the prospectus, which explains what the business does, shows its financial results and lists the risks it sees.

A real example: on Monday, Oct. 26, Anthropic, the AI company behind Claude, publicly filed its S-1 and said it plans to list on the Nasdaq Global Select Market under the ticker "ANT." CNBC reported that it first filed confidentially in June, which lets the SEC review the numbers before they're made public. The filing doesn't set a price yet. Anthropic said the number of shares and the price range "have not yet been determined," and no shares can be sold until the SEC declares the registration effective. Reuters said the news helped lift AI and chip stocks that day, and the S&P 500 closed up 0.70% at 7,676.77.

A few beginner points: a filing is a step, not a start date. The usual order is the filing, then a price range, then a final offer price, then the first trade on the exchange. The offer price is what buyers in the offering pay, and the price once regular trading starts can be very different, so new listings can swing a lot in their first days. The "Risk Factors" section of any S-1 is worth a read. Questions about IPOs, tickers or prospectuses? Ask below. There are no silly questions here.

Sources: Anthropic announcement and SEC Form S-1 (Oct. 26, 2026); CNBC Anthropic IPO live updates (Oct. 26, 2026); Reuters via Yahoo Finance market close (Oct. 26, 2026); Investor.gov "Investing in an IPO" investor bulletin.