The y-intercept is the y-coordinate of a point where a line intersects with the y-axis.
The green line on top crosses the y-axis at (0, 5) while the one at the bottom crosses the y-axis at (0, -3)
For the point (0, 5), 5 is the y-intercept.
For the point (0, -3), -3 is the y-intercept.
Notice that the y-intercept is always located on the y-axis.
Notice also that the x-coordinate is always equal to zero when a point is located on the y-axis.