The y-intercept of a line is the point on the y-axis that is contained in the line. It is the point of intersection of the line and the y-axis.
Every point on the y-axis has coordinates (0, y), where y can be any real number. The x-coordinate of all points on the y-axis is 0. Since you are told your line goes through point (0, -6), then you know that that point is the y-intercept.
