What exactly is toxic shame?

Toxic shame. Shame is the intensely painful feeling or experience of believing that we're flawed and therefore unworthy of love & belonging. It's "toxic" because it often leads to hiding or withdrawing -- which intensifies it. Other results can be trying to overly please or ingratiate others, or getting aggressive towards others to gain power by shaming them. Guilt = "i did something bad." shame = "i am bad.".
Harmful shame. Shame is a normal human emotion. It's sometimes confused with guilt and hard to distinguish the difference between them. We do things & suffer things that cause us to be ashamed. Guilt is mostly from our own bad behavior. Shame that we don't deal with successfully can bring depression & hopeless helpless feelings & demoralize us. That's toxic physically and emotionally. Best to talk about it.
Shame. As I hear the question, I would say shame in general has different levels of toxicity. By this I mean people often feel shame because of being abused by authority figures or peers who tell the person they should be ashamed, when oftentimes there is no reason whatsoever. A way of thinking about shame for legitimate reasons is to think of everyone having something to be ashamed about as a rule.
Shane. Another view of shame. I just emphasize that this is not a settled question. Shame can be thought of as a necessary feeling. We then learned to have thoughts about that feeling , our own personal reaction to that feeling. I have learned that it is the feeling that we all get when we can't get what we want. Reaction to that moment , when we feel hurt, is learned and can be unlearned.