I would define financial security as the ability to pay for your desired lifestyle with minimal perceived risk. Some risk always exists, but it's your perception of it that either puts you at ease or stresses you out.
Until a year or so ago, a cushy career at Facebook seemed like a pretty sure thing and the opposite of risky. To others, any single source of income is a massive risk.
Owning a house, for instance. Having a solid job or a good resume so you can always find a job if you don't like your first, or own your own business that does well so you cannot lose your job and can live freely. Connections so that your business (or job quest) has a good chance of getting good results. A good income stream so you don't run out of money.
A lot of it is centered around money, such is our society.
A lot of things are fleeting, so it is important to also live in a country where the government won't take everything away from you because they don't like your opinion, or living in a country where war is less likely. (There are ways to mitigate these risks).
Until a year or so ago, a cushy career at Facebook seemed like a pretty sure thing and the opposite of risky. To others, any single source of income is a massive risk.