So, imagine you and your friend were playing cops and robbers. Your friend was the robber and you were the cop. You both had toy guns and were running around having fun. However, at one point, your friend accidentally hit you with his toy gun and it hurt. You became upset and decided that your friend was no longer allowed to play the game because he wasn't following the rules.
This is kind of like what happens with the term "enemy combatant." During a war or conflict, some people might be fighting against your side. If they follow the rules of war and don't target civilians or use weapons that cause unnecessary harm, then they are considered to be enemy soldiers or fighters. However, if they break these rules and kill civilians or use weapons that cause unnecessary harm, then they might be labeled as "enemy combatants" instead.
Now, the term "no longer enemy combatant" means that someone who was previously labeled as an enemy combatant is no longer considered one. This could happen if they stopped fighting against your side, surrendered, or if it was discovered that they were never really involved in the conflict to begin with. It's like if your friend who accidentally hit you in the game apologized and promised to follow the rules from now on - you might decide to let him back in the game because he's no longer breaking the rules.