The anterior humeral line is a line drawn down the anterior part of the distal humerus. Normally this line should pass through the middle or middle third of the capitellum on the true lateral view of the elbow. When a fracture occurs in children, it is usually a supracondylar fracture through the weakest part of the humerus (note the wine glass shape) and this results in posterior displacement, moving the capitellum backwards.
The first radiograph above is a normal lateral view. The second demonstrates a supracondylar fracture with posterior displacement of the capitellum - zoom in to see the fracture more clearly.