It is perhaps best described as having a low opinion of oneself (either consciously or not), and/or feelings of being 'worthless'.
It can surface in thoughts and in feelings and will often appear to manifest physically - in body postures, actions and health.
All of this can result in:
-Feelings of 'being stuck'
-Low motivation
-Low energy levels
-Lack of physical activity
-Potentially destructive behaviors
-Depression
-Feeling helpless to change anything
-Relationship difficulties: Low self esteem and divorce are unhappy partners.

Self esteem: How to improve it
This can get tricky as any change requires a certain amount of motivation - something those those experiencing low self worth may not have much of.
And just trying to be a cheery and positive person around them can even make it worse. I'm sure we have nearly all experienced the "Cheer up, it may never happen!" type of 'advice!