You might find these definitions from Black's Law Dictionary useful in understanding this principle. The judge would explain to the jury the definition of a "legal person" for the purpose of the suit.
"CORPORATION
An artifical person or legal entity which is regarded in law as having a personality and existance distinct from that of its several members, and which is, by the same authority, vested with the capacity of continous succession, irrespective of changes in its membership, and of acting as a unit or single individual in matters relating to the common purpose of the association, within the scope of the powers and authorities conferred upon such bodies by law.
DOCTORINE OF CORPORATE ALTER EGO
Means that courts ignoring forms and looking to substance will regard stockholders as owners of corporate property, or as the real parties in interest whenever it is necessary to do so to prevent fraud which might otherwise be perpetrated, to redress a wrong which might otherwise go without redress, or to do justice which might otherwise fail."
Simply put, the assets of the owners (stockholders) are subject to confiscation by the court to pay awards ordered by the court for actions taken by the company's officers or employees in the performance of their jobs.
Have you ever heard of a lawsuit against Exxon for the actions of a ship captain in their employ who got drunk on duty and ran the ship aground? Exxon was liable for the actions of that captain even though he violated the law and company policy and Exxon had to pay.