Question

What SI unit replaces the pound. what does this measure

Answer 1

The SI system of measurements was created to provide a standard
system that could be used and understood by scientists and ordinary
people everywhere.  No such system had existed before.  The SI ('metric')
system stands on its own.  It doesn't explicitly 'replace' anything, unless the
users of other systems decide to throw off their ancient, complicated, and
difficult systems and climb aboard the metric bandwagon.  To date, every
country in the world has done that, except the three backward shadowy
societies who prefer to resist change and wallow in their old habits. 
The three holdouts are Burma, Liberia, and the USA.

In the complex and confusing "customary" system, the "pound" is used
both as a unit of mass and as a unit of force. 

When "pound" means "poundmass", it can be replaced by the kilogram. 
And when it means "poundforce", it can be replaced by the Newton.