JL is a side that is common to both triangles. It is congruent to itself.
The diagram marks side JM as congruent to side JK (using a single hash mark).
The diagram marks side ML as congruent to side KL (using a double hash mark).
Thus, we have JL ≅ JL, JM ≅ JK, and ML ≅ KL. Three sides of one triangle are congruent to three sides of the other triangle, so the SSS postulate applies. We only need to make sure that the sequence of letters describing the two triangles is appropriate.
We know that J corresponds to J, L corresponds to L, and M corresponds to K. This means when we describe one triangle as ∆JML, we must describe the congruent triangle as ∆JKL. (Corresponding letters are in the same order.) That is how the problem statement describes the two triangles, so we conclude ...
... ∆JML ≅ ∆JKL by the SSS postulate