Foreign travelers visiting the United States are required to hold passports that are valid for six months beyond the period of their intended stay in the US. However, citizens of the countries listed below (six month club) are exempt from the six-month rule and need only have a passport valid for their intended period of stay.
So essentially, the United States considers these countries' passports valid for additional six months after their expiration date.
If your country does not have such an agreement with the United States, and your passport doesn't meet the six-month requirement at time of entry, your authorized stay may be shortened accordingly. So if at all possible, renew your passport early to avoid any potential issues when entering the U.S.
Note: U.S. Customs and Border Protection (CBP) updates the six month club list on a regular basis. It is thus important that you check their website (www.cbp.gov) for the latest version before making travel arrangement.