White’s queen is aimed at a diamond-shaped cluster of Black material. It looks hard to make headway because the bishop on e5 seems solidly guarded. But notice a great weakness in the bishop’s position: it is flanked on both diagonals, by a Black pawn and queen. It therefore has no way to retreat, and is vulnerable. (This visual pattern is worth remembering.) White thus can create great trouble by simply attacking it with a pawn: d2-d4. No piece, no matter how well protected, can sit still when threatened by a pawn. Since the bishop has no safe flight squares, Black is obliged to play Bxd4 (well, “obliged” is a bit strong; more on this below). In itself this gains White nothing, but look at the changes that have been forced on the board: once Black’s bishop steps forward to d4 it is attacked once (by White’s bishop on f2) and guarded once (by its queen on d6). And the Black bishop’s movement also has opened a line from White’s queen to the knight on e7—which also is guarded only by Black’s queen. The Black queen has become overextended, so the position invites White to play one capture or the other. Which should it be? Clearly not QxN, since it loses White’s queen for a knight. No, White starts with the cheaper piece and plays BxB, gaining a piece for a pawn. (If Black recaptures QxB, then of course White has QxN.)
As suggested a moment ago, Black doesn’t have to reply to 1. d2-d4 with Bxd4. He can forfeit the bishop and try to improve his situation in other ways, as by playing his knight to g6 so that after 2. d4xB Black can use the knight to recapture. Black still has suffered a loss, and indeed has a lost game, but at least he ends up with a nicely-placed, well-defended knight on e5.
The point: overworked pieces sometimes can come into view unexpectedly, after some initial threat or capture and your opponent's reply to it. Practice asking not only how his pieces are defended, but how they will be defended after the changes you can force.