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The horizontal blue line connects two points where the red layer is at the same elevation (i.e., crosses the same contour twice).  You can see that the layer dips in a direction perpendicular to this line. 

The horizontal line on such a layer is called the strike; it can be determined by drawing a line connecting points where the layer crosses the same contour.  The strike direction is measured from true north, such as N45°W for a plane that has a northwest strike. In our example, the plane has an east-west strike. 

The dip of a plane is the angle that a plane is inclined from the horizontal: a horizontal plane has a dip of 0°, a moderately dipping plane has a dip of 45°, and a vertical plane has a dip of 90°. In our example, the plane has a gentle dip (between 0° and 45°).