Wali al-ahd is a fancy word that means "heir apparent" in Arabic. In simpler terms, it's like when you see a big brother or sister who is older than you and someday they will become the leader of the family – that's kind of what it means.
In some countries, they have a special system where the leader (like a king or queen) picks someone to be their wali al-ahd, which means that person will become the leader when the current one is no longer able to lead. It's like picking a "successor," or someone who will take over a really important job when the current person is finished doing it.
Think of it like playing a game of soccer. If the captain of the team gets tired or sick and can't play anymore, someone else on the team who has been practicing and is really good might become the new captain. The wali al-ahd works kinda like that, but instead of soccer, it's leading a whole country!