Flood insurance
Flood insurance is a separate policy that pays for flood damage, which standard homeowners policies do not cover. You can buy it through the National Flood Insurance Program or from a private insurer.
New Jersey example
A Little Falls homeowner who buys flood insurance the week a storm is forecast likely won't be covered for that storm, because there is normally a 30-day waiting period before a new flood policy takes effect. The homeowners policy won't pay for the flooding either, even if the flood comes with a hurricane.
Official source
NJDOBI | Consumer Information - Preparing for Extreme Weather
This is a plain-English summary. Rules and dollar amounts change, so check the official source or a professional before you file or sign anything.