The fear that we are not good enough. We often feel unworthy of love, to dirty for people to want to be around, not deserving, and not good enough too belong.
Shame tells us to hide (it isolates us) and that we are not worthy of love and connection. That we are bad people.
Go deeper, see You Are Not Meant To Live in Shame, post 1 and There Is No Shame In Feeling Shame, post 2.