The graph of x = |y| is as shown in the figure.
When we draw a vertical line, it intersects the graph at two points.
This means for the same value of x , we have two values of y.
Or in other words, for the same pre image, we have two images.
Hence it is not a function.
But for every value of x, we do have a value of y, hence it is a relation.
x = |y| is a relation but it is not a function.