Ancestry.com became a publicly traded company on NASDAQ (symbol: ACOM)
on November 5, 2009, with an initial public offering of 7.4 million
shares priced at $13.50 per share, underwritten by Morgan Stanley, Bank
of America, Merrill Lynch, Jefferies & Company, Piper Jaffray, and BMO
Capital Markets.[26]

https://en.wikipedia.org/wiki/Ancestry.com

What does underwritten mean here? I see it all the time wrt IPOs.

Underwriters are used for insurance because someone has to agree to pay
if there is a large claim or a slew of smaller claims.

But IPO's take in money. What are those 4 companies underwriting, what
are they doing?

Why can't some bookkeeping company with no assets just sell the stock
and take in the money for ancestry.com?