It isn’t that hard. Being transgender means that a person’s gender doesn’t (entirely) match the gender they were assigned at birth. That’s all there is to it.
It’s hard for people with no experience of being transgender to understand what that is like, and many people have a hard time believing it’s possible to feel that way. Or they might think it’s possible but not understand the nuances of that experience in someone’s life.
How do you describe colors to someone who has been blind since birth? How do you describe the sound of a baby crying to someone who has been deaf since birth? It’s tough for someone who hasn’t experienced even the primary sensations of gender incongruence (or even sensed their gender) to grasp the concept, much less how it must feel to live with those feelings all the time.
This is a case where language fails us. We don’t have good language to share what gender feels like, much less what it feels like when it conflicts with one’s sex and/or how others perceive them. So, we do our best to muddle along and give our best effort but ultimately inadequate metaphors and analogies.
If you need to see proof of this in real life, watch two trans people talk to each other about how they’re feeling - when one says they’re feeling dysphoria, the other is immediately receptive and comforting. In contrast, when I share that I’m feeling dysphoria with even the most loving and well-meaning cisgender people, I often get blank stares or questioning looks. It must be experienced to be understood, and once it’s experienced, no explanation is necessary.