I've been reading about it lately. A financially secure person is someone who basically can live (frugally) from his investments, have many sources of income, etc.
If you're a top notch laywer then you're wealthy but not financially secure (if that's your only income).
On the other side, if you have a moderate income, but you have many sources of income (e.g. rental units) then you're FS.
The book "The millionare next door" is perhaps the best book if you want to learn about this. The only problem I have with the book is that it doesn't consider the quality of life among the cases it presents.