Low self-esteem is usually accompanied and treated with other conditions, often depression, an anxiety disorder, shame or lack of assertiveness. It means a negative sense of self and might be an underlying sense of worthlessness that is hard to categorize or might be accompanied by specific thoughts such as "I'm a failure" or "I'm stupid".
When someone is depressed and has low self-esteem, it might be that the low-self esteem leads to the depressed mood, it might be that the low-self esteem is a result of being depressed or the two might be intertwined. Both need treating.
People who have low self-esteem are often highly self-critical (and assume that other people are too) and can be punitive towards themself and others.
Treatment is by correcting the erroneous cognition backed up by behavioural experiments outside the sessions to confirm the new thinking.