In any rhombus, the four sides are the same length.
Because of this, we know that
Furthermore, AC = AC due to the reflective property
Those three facts then allow us to prove triangle ACD is congruent to triangle ACB through the SSS congruence property. Then from CPCTC, we then know that angle DAC = angle BAC, which is enough to show that AC bisects the angle shown. The term "bisect" means "cut in half".
