This progression plays a major chord, adds a 7 note to the chord (which is a half step lower than the root note), and then adds a flatted seven to the chord. What is happening is a three step descending progression: the root note is flatted and added, and then the note is flatted again. The I – Imaj7 – I7, is often followed by the IV chord. An example of this progression is in the song “Give Me Words to Speak” by Aaron Shust (I believe he does this song in the key of A).
The progression in the key of D would be followed by a G:
Listen to this progression in D:
The progression in A would be followed by a D:
Listen to this progression in the key of A:
The progression in C would be followed by an F:
Listen to this progression in the key of C: