Idle speed is controled by air let into the engine. The throttle plates are set fully closed with your foot off the throttle so to allow the engine to idle, Ford uses an Idle Air Control device. This is a device just past the throttle plates on the throttle body that has a solenoid that drives a valve that can be controlled with fine movements. This bleeds air into the engine to maintain the appropriate idle speed regardles of engine load.
A low idle implies a few possible issues:
A) The engine computer doesn't realize the engine idle is low
B) The engine computer is trying to use the IAC to adjust engine idle, but the IAC is not responding properly.
C) The engine computer is using the IAC, but the IAC is at the max of it's opening and still not enough air is coming in.
#B is the most common, and might be due to a sticky valve. You can try to remove the IAC and clean it with sensor safe throttle body cleaning spray. You need to be careful though not to use too much pressure as it can damage a diaphram inside the sensor. Best to soak the non-electrical portion of the unit in the cleaning solution to let it sit, and then see if that works.
If that doesn't work, it may need to be replaced.