It says that the measure of the intercepted arc is twice that of the inscribed angle. The precise statements of the conjectures are given below. In a circle, two inscribed angles with the same intercepted arc are congruent. The measure of each inscribed angle is exactly half the measure of its intercepted arc. Since they have the same intercepted arc, they have the same measure.
Any angle inscribed in a semi-circle is a right angle.
The intercepted arc for an angle inscribed in a semi-circle is 180 degrees. Therefore the measure of the angle must be half of 180, or 90 degrees. In other words, the angle is a right angle.