The Flash Flood watch is in effect for  portions of Delaware, northeast Maryland, southern New Jersey, and southeast Pennsylvania. The Flood Watch is for – Delaware, Eastern Chester, Eastern Montgomery, Philadelphia, Western Chester, and Western Montgomery. In Delaware – Delaware beaches, inland Sussex, Kent, and New Castle. In southern New Jersey – Atlantic, Atlantic Coastal Cape May, Camden, Cape May, Coastal Atlantic, Coastal Ocean, Cumberland, Gloucester, Northwestern Burlington, Ocean, Salem, and Southeastern Burlington.
According to 6ABC AccuWeather

Light to moderate rainfall will develop across the area this afternoon. The rainfall is expected to become heavier later this evening into the overnight hours. A total of one to two inches of rain are expected across portions of the area. some thunderstorms may also occur and could be slow moving which could lead to locally higher amounts.

Excessive rainfall within a short period of time can lead to rapidly rising waters and flash flooding, particularly in urban areas and along small creeks and streams.
A Flash Flood Watch means that there is the potential for flash flooding which can be life-threatening. Heavy rain is expected to occur over a short period of time. Rapidly rising flood waters may quickly inundate roadways and areas of poor drainage. Streams and creeks could leave their banks, flooding nearby properties. Please monitor the forecast, especially if you live in a location that is prone to flooding. Be prepared to take action if a flash flood warning is issued for your area.