What Does The Built-In-Function Faces() Do?

Faces is a top-level API (similar to Instance) used when working with Handles objects.

If there, as an example, was a part that you only wanted to allow someone to move vertically using a Handles object, you could write:

local handles = Instance.new("Handles", workspace.Part)
handles.Faces = Faces.new(Enum.NormalId.Top, Enum.NormalId.Bottom)

Here is an example of what that looks like

See here for a description of the Handles object.