An edge is simply a line connects two vertices together. Using the two vertices, you can easily find the edge’s formula, as well as its direction, and other properties. Since the edges don’t contain special data and they’re used not so often, Unity doesn’t store it anywhere, but you can refer to them indirectly via the vertices and triangles arrays.
We almost don’t care about edges during the mesh generation process, but it doesn’t mean that they are useless. One example for their application is to manually calculate the normal vector of a specific face, by using cross product of the two edge’s direction.