Two sides have the same length, which is less than the length of the third side.
The two sides they are referring to are the legs of the triangle, and the third side is the hypotenuse. We know that the legs are congruent because their base angles are congruent (45 and 45). We also know that they are both less than the third side because the hypotenuse is always the longest side in a right triangle. Hope this helps!