What is the “minority attack” w respect to queens gambit declined?

If you look at the exchange variation, White has no c-pawn, Black has no e-pawn.

So White has 2 queenside pawns, 2 central pawns, and 3 kingside pawns.

Black has 3 queenside pawns, 1 central pawn, and 3 kingside pawns.

So when a side attacks with fewer pawns on the side of the board than his opponent, it is called the Minority Attack.  His 2 pawns attack Black's 3 pawns on the Queenside.  This is just the most common form of the minority attack.  A minority attack can be 1 on 2, 3 on 4, etc as well.

In a minority attack there is normally a blocked pawn at top of the pawn chains. This is important because if you advance your pawns to attack the majority the pawn would be able to move forward making passed pawn.

A minority attack is when you have , say, 2 pawns on the queenside vs your opponent, who has 3. You push your two pawns to create open lines and attack his majority.

Basically, white runs queenside pawns up and saddles black with a backwards c6 pawn.