I hear the word insecurity used frequently to describe the motivations for different, often completely opposite, behaviors. Sometimes it is used synonymously with low self-esteem or a lack of confidence, but I think it is often used in black and white terms, such as, this person is insecure and that person isn't, whereas much of it is circumstantial and may not even be consistent within the same person. Everyone has strengths and weaknesses, and sometimes the more pronounced a person's syrengths are, the more pronounced their flaws are. A truly secure person doesn't deny their weaknesses, but learns to manage them with their strengths to create a healthy balance. Too often people, men in particular, are encouraged to hide their weaknesses and the result is a deeply insecure and delusional individual.
Oftentimes, certain traits are viewed as indicators of insecurity when it may be the exact opposite. A person who is quiet natured and contemplative doesn't necessarily have low self-esteem. Perhaps, they are self-contained and content and feel no reason to draw attention to themselves. On the otheer hand, perhaps they are timid because they fear rejection or criticism. A lot of times, people who are loud, outgoing, and socially suave are viewed as having high self-esteem, though plenty of people exhibit these traits because they are seeking validation from others and are not self-content. By that, I don't mean in regards to companionship, but a need for social status. That's just one example of how opposite behaviors can indicate self-security in one person and insecurity in another.
It makes sense that a person who is approaching unfamiliar territory would be a bit apprehensive or reserved until they have a sufficient reason to proceed with certainty and the utmost confidence, but I've heard the term "insecure" used in reference to people who exercise more caution than carelessness.
I think people can be sufficiently confident in one aspect and less secure in another without the word "insecure" being entirely applicable. I see it more as a sliding scale. Im wary of the person who constantly needs to assert their happiness, success, status, etc. If they were truly satisfied then they wouldn't need to broadcast it and seek validation.
I hope this made sense. There were a lot of ideas thrown in there and I typed it on my phone.