What is Taklif (Obligation)? Who is called a Mukallaf?

Taklif means to order someone to do something that necessitates sacrificing, to enjoin someone to do it. 

As a term, taklif means: The religion of Islam ordering people to obey Allah’s orders and to avoid His prohibitions.

A person who is obliged to act in accordance with Allah’s orders and to perform worshipping is called a mukallaf (religiously responsible or accountable).

The world we live in is called the place of taklif because it is the place where people are made responsible for the orders and prohibitions of Allah and for worshipping.