Lots of people are fooled by the name.  Salt & Light are not
normally concepts that are linked, so why link them now?
                    

Not so difficult  - if you know the answer!

It's from Matthew's Gospel in The Bible.

Chapter 5 verses 13 - 16
 
 You are the salt of the earth... You are the light of the world. 
 A city set on a hill cannot be hidden.  Neither do people light
 a lamp and put it under a bowl.  Instead, they put it on its
 stand, and it gives light to everyone in the house.  In the 
 same way, let your light shine before men, that they may see 
 your good deeds and praise your Father in heaven.
                      
 Why?

The idea is that Christians should be like salt, preserving the
good things in life, and like light, bringing joy and clarity to 
people's lives, so that people will see the good things  we do and give God praise.
  

Also, The Salt & Light Company idea was used by a
band called Ishmael and Andy, back in the mists of time. 
I loved their songs, loved the band and recycled the expression.