This is the first target he gets to attack, and it can be either a train or a train yard. If the target is a train yard, it can only be destroyed by an Explosion and not by accumulated damage. Once the Zug card is revealed, opposing players may play Action cards in attempts to foil the player's attack. You may find a German fighter on your tail, or a train may round a hill or enter a tunnel, or clouds and fog might move in on you. Assuming you get past these hurdles you attack the target with a selected ordnance of your choice.
Bombs always cause Explosions if they hit, and Explosions always destroy any target.
Rockets and strafing usually inflict damage points which only affect trains, not yards, but both have a chance of causing a target-rending Explosion as well. If you cause an Explosion, or you inflict the necessary damage points, the target is destroyed and you keep the Zug card and get the number of Victory Points indicated on the card. Assuming you have fuel and ammo remaining, you then flip the next card in the Zug deck, expend another fuel card, and repeat the process. If the target survives your attack you can keep attacking it until you have no more fuel or ammunition, at which point your turn ends. An undestroyed train yard stays in play and becomes the next player's target, while undestroyed trains escape and are set aside until the next round starts.