A person who is insecure lacks confidence in their own value and capability. This is not to be confused with being humble, which involves recognising one's failings but still maintaining a healthy dose of self-confidence. Insecurity is not an objective evaluation of one's ability but an emotional interpretation, as two people with the same capabilities may have entirely different levels of insecurity.
Insecurity may cause shyness and social withdrawal, or alternatively it may encourage compensatory behaviours such as aggression or bullying, a principle enshrined in the phrase "all bullies are cowards" (this saying is in fact fallacious, as some bullies are not cowards, see psychopath). Many people suffer a period of insecurity during puberty, which gives rise to a lot of the stereotypical behaviours of adolescents.
See also Uncertainty.