The x-intercept is the x-coordinate of a point where a line intersects with the x-axis.
The green line on the left crosses the x-axis at (-6, 0) while the one on the right crosses the x-axis at (4, 0)
For the point (-6,0), -6 is the x-intercept.
For the point (4,0), 4 is the x-intercept.
Notice that the x-intercept is always located on the x-axis.
Notice also that the y-coordinate is always equal to zero when a point is located on the x-axis.