Student or Learner
Can you explain the meaning of it?
Any sentence to show the usage would be very appreciated.
Usually it would mean the person who has the skill to make a company (or any organisation, really) work, and work well. This is not necessarily a person who is actually in charge!!
Here is an example:
The local volunteer group has many willing members, and a capable chairman, but it is Bob who is the brain(s) of the outfit. He's the one who makes things work.
The small family company was owned and managed by the father, but it was the son who was the brain of the outfit; he's the one who really made the business profitable.
Hope this helps!