A more thorough explanation:

A free writing prospectus (FWP) is a document that a company releases during the waiting period of an initial public offering (IPO) to provide additional information about the securities being offered. This document is different from the registration statement and can include information that is not included in the registration statement.

According to Rule 405 of the Securities Act, a free writing prospectus is any written offer to sell securities made by means other than a final prospectus, a preliminary prospectus, a written communication made in reliance of Rule 167 or Rule 426, or any written communication delivered together with the final prospectus.

For example, if a company is planning to go public and wants to provide additional information about the securities being offered, they can release a free writing prospectus. This document can include information about the company's financials, management team, and other details that are not included in the registration statement.

However, issuers must comply with Section 10 of the Securities Act when making written offers during the waiting period. Rule 164 states that a free writing prospectus that satisfies the requirements of Rule 433 will be considered a prospectus under Section 10, and therefore an acceptable written offer. To satisfy Rule 433, the free writing prospectus must contain information that is not included in the registration statement but does not conflict with the registration statement.

Issuers often use a free writing prospectus when they make changes to their IPO, but their roadshow already began. Additionally, Rule 163 allows well-known seasoned issuers (WKSIs) to make free writing prospectuses during the pre-filing period.