Cone of vision in perspective drawing
I am no expert, but a simple Google search gave me a very clear answer. The cone of vision is the spatial volume seen by a (dimensionless) observer inside an angle of 60o.

To "calculate" the envelope (margin) of the cone, you "draw" all the lines originating at the observer and being at an angle of 30o away from the line of sight.

Everything outside this cone is considered "out-of-sight".

Notes:


For humans (and most living creatures possessing vision), there are two distinct cones of vision, one for each eye. The relative position of the two  cones varies greatly, depending on the relative position of the eyes (e.g., both eyes to the front, like humans, or eyes to the sides, lie fish). Eye movement leads to movement of the cone of vision.
For how to successfully use the concept of "cone of vision" in drawing, architecture or anything else, you need to get further knowledge on all these subjects.