Basically security means freedom from danger, from fear or anxiety, from want or deprivation.
Basic, almost primordial, fear and insecurity has driven humans to become tool makers and engineers. That is: reducing the uncertainty and building predictability and safety into what was experienced as a harsh and dangerous environment.
In essence security applies most at the level of the citizen. It amounts to human wellbeing: not only protection from harm and injury but access to water, food, shelter, health, employment, and other basic requisites that are the due of every person on earth. It is the collectivity of these citizen needs - overall safety and quality of life - that should figure prominently in the nation's view of security.
Anyone who trades liberty for security deserves neither liberty nor security (Benjamin Franklin).