Hypotenuse-Leg (HL) is a special congruence theorem that only applies to right triangles. If you have two triangles where you know
a) One pair of hypotenuses are the same length
AND
b) one pair of legs are the same length
then this means we can prove the right triangles congruent. This theorem does not work for non-right triangles (if non-right triangle, then this is the SSA case that doesn't work all the time)