White looks at everything his pieces attack and sees an apparently attractive opportunity: a loose pawn on e7 that he can take with his queen. Any problem with this? The capture is a disaster for White because now he has done Black the favor of clearing the e-file of everything except White’s own queen and rook. A queen and rook on an open file can form a powerful battery and serve as part of an effective mating attack, but it also can serve as the kernel of an unwelcome pin or skewer. The vulnerability of the pair depends in part on the quality of the protection the pieces enjoy. Here White’s rook on e1 has no protection on the first rank, and indeed no protection from any piece but the queen—a characteristic of many targets of skewers. Black thus is able to reply with Re8, where the rook forces the queen out of the way with protection from the knight on f6 as well as the rook on a8. The queen has no square to which it can retreat and keep protecting the rook on e1, so Black will take the rook next move. Or White will have to let his queen go and recapture with the rook. The lesson: before making any capture—even of a loose piece or pawn—consider whether you are aligning your pieces in troublesome ways that would allow a skewer or pin by your opponent.
The actual best move for White here is Nd2, which enables him to threaten Qxe7 for real; moving his knight out of the way like this connects the rooks on his back rank, allowing them to defend each other and thus immunizing the e1 rook—and the queen—from the threat of a skewer.