TL;DR An IPO (initial public offering) is a private company selling shares to the public for the first time — the day it becomes a stock. Insiders and institutions get the offer price; you usually get whatever the hype costs.
The plain-English version
Private companies are owned by founders, employees, and early investors. An IPO opens the doors: the company (with investment banks as organizers) sells a block of shares to the public and lists on an exchange. From that day, anyone can own a piece, the price updates every second, and the company files public financials forever after.
Why go public? To raise money for growth, to let early backers finally cash out, and to make stock-based pay real. In exchange, they accept life under a microscope.
How it actually goes down
Banks help set an offer price and sell those shares — mostly to big institutional clients — the night before trading. Then the stock opens to the public, at whatever price supply and demand produce. That gap matters: when a stock "pops 40% on its IPO day," the 40% went to whoever got offer-price shares. Regular buyers paid the pop.
The other date to know: the lock-up expiration, typically ~90–180 days after the IPO, when insiders become free to sell. Supply can flood in; the date is public; the market rarely forgets it.
The track record nobody prints on the balloons
IPO day is a marketing event executed by professionals whose job is a high price. Historically, plenty of hyped IPOs traded below their first-day prices within a year — buying day one means paying peak attention. Some IPOs are generational companies; you usually couldn't tell which ones from the confetti. There's also the quieter cousin, the direct listing (no new shares, no banks pricing the deal) — same public debut, less machinery.
The common mistake
Confusing a great company with a great first-day price. Those are two separate questions, and the second one is the only one you're being sold.
Educational only — not investment advice. On IPO day, ask who's selling — and why today.