A more thorough explanation:

Vis Proxima means "proximate force" in Latin. It refers to immediate force. For example, if you push a book off a table, the force you use to push it is vis proxima. It is the force that is directly applied to the object. Vis Simplex means "simple force" in Latin. It refers to mere or sheer force. For example, if you hit a nail with a hammer, the force you use is vis simplex. It is the force that is applied without any other factors affecting it. In summary, vis proxima is the immediate force applied to an object, while vis simplex is the sheer force applied without any other factors affecting it.