Self-loathing is when someone feels very, very bad about themselves. It's like when you do something really mean, accidentally break something important, or get in trouble for something you didn't do. You feel really bad and maybe you start to think that you're not a good person.
Sometimes, grownups who have problems or who have been through hard things in their lives might start to feel this way too. They might start to believe that they're not smart or not pretty or not good enough for other people to like them.
But, just like when you make a mistake or do something wrong, it's important to remember that everyone makes mistakes and nobody is perfect. It's okay to make mistakes and to learn from them. You are still a good person and you still deserve love and kindness, even when you do something wrong.