You know this bc the markings on the drawing show that Angle B and Angle D are congruent. Also that AB is parallel to CD. This means that Angle BAC and Angle DCA are congruent by ALTERNATE INTERIOR ANGLES. Also, AC is congruent to itself. That makes the two triangles congruent by ANGLE-SIDE-ANGLE.
Now, you need to know that "congruent" triangles (and shapes in general) are the same size and shape. "Similar" triangles (shapes) are the same shape but not necessarily the same size.
