Face is a closed set of coplanar edges. A single face often contains three or four edges, but in some case it can be more, depend on the render engine you use. A face with three edges is called a triangle, and the one with four edges is called a quad. In Unity, most of faces are triangles.
Face is the element makes the mesh visible. In Unity, the process of rendering a face is broken down to 2 steps: Render front face and back face. So how can we determine which side is front face and which side is back? To form a triangle, you have to pick 3 vertices into a group, the order matter! When looking down to the mesh from the camera, front face is the side which its vertices order goes clockwise, and back face is the side which its vertices order goes counter-clockwise. However, it doesn’t matter which vertex you start with.
In Unity, triangles are simply stored as an integer array, where each element is an index to the vertices array. Each triangle will take 3 indices; thus the array length will be a triple of number of vertices.