They're asking for what other information is needed.
We were given triangle ABC is congruent to triangle DEF. We know for certain line AB and line DC are congruent to each other as well as lines BC and EF.
The additional information we need to know is if line AC is congruent to line DF to absolutely be sure.
Remember SSS by definition is: Triangles are congruent if all three sides in one triangle are congruent to the corresponding sides in the other. If all three sides in one triangle are the same length as the corresponding sides in the other, then the triangles are congruent.
