Its a type of filing. Confidentially means that we cannot go so EDGAR and look up the filing right now, and they will not need to disclose the documents publicly until very close to the actual IPO. This is a relatively new type of filing that was introduced in the JOBS act, designed to encourage emerging businesses to go public. You can read about the specifics here:
https://www.sec.gov/divisions/corpfin/guidance/cfjumpstartfa...
Normally, if the company does not qualify for confidential filing, we would be able to go and see the draft S-1 on EDGAR 6 - 9 months in advance of the actual IPO.