The winding order of meshes in Unity is clockwise.
Winding order produces the face normal, the direction a face is looking. The order of the triangle's indices that look up into the vertices array define the winding order. When viewed head-on, the front face's indices should be ordered in a clockwise fashion.
Triangle indices are expressed as integer values that point to the vertex attribute arrays, and are in groups of three. Vertices can be reused by the triangle indices.
Unity, huh, how?
vertices: [..., (0, 1), ..., (1, 0), (0, 0), ...]
indices: 12⤴ 21⤴ 22⤴
In this diagram of a triangle the triangle indices must be 21, 22, 12, 22, 12, 21, or 12, 21, 22.
Other orders would produce a counter-clockwise winding order and the face would not render to the screen from this direction.