A selling stockholder is a current owner who sells some or all of their shares in an offering, usually a secondary one. The prospectus names them in a table, with how many shares each owned before the sale, how many they are selling and how many they will hold afterward. They can be founders, venture funds, other institutions, executives or employees.
The company does not receive proceeds from these shares, though it normally bears the expenses. Sellers also take on legal responsibilities for the information provided about themselves.
After an IPO, most existing holders are barred from selling for a period, commonly around 180 days, under lock-up agreements with the underwriters. When the lock-up ends, a large number of shares can become eligible for sale, which can affect the price.
Read the selling stockholder table, note who sells and who stays, and check the date when the lock-up ends.