Flatpicking is a technique for playing the steel-stringed acoustic guitar using a plectrum or flatpick. Although the term is used in other genres, it is probably best known as the lead guitar style in bluegrass music and old-time country music.
Flatpicking involves holding a plectrum with only your thumb and index finger to play the lower notes while picking the upper notes with your middle and/or ring fingers.
In the column entitled Tricks of the Trade there is a particular riff on “Knocking On Heaven's Door,” which can be played many ways. The author tends to fingerpick although using a plectrum is another possibility.