What is single equivalent force?

It typically means translating all the external forces and torques acting on a body into a single equivalent force acting at a particular point and direction on the body.

Can you give me some write ups to elaborate this topic... and thank you for replying so soon :)

A quick Google search will give you several write-ups. Here's one to get you started:

Equivalent force systems (http://emweb.unl.edu/negahban/em223/note8/note8.htm)