Okay kiddo, so there are different groups of Muslims, and they sometimes have different opinions about how to follow their religion. Al-Azhar is a really old and respected group of Muslim scholars in Egypt. Fatwa is a legal ruling or opinion by these scholars on a particular issue, sort of like a judge's decision. Shia is one of the two main branches of Islam, and they have different beliefs from the other branch called Sunni.
Recently, there was a fatwa (ruling) by some shia Muslim scholars that said it's okay for men to marry young girls who haven't reached puberty yet. This caused a lot of controversy and upset many people, including some other Muslim scholars who said this is not acceptable. Al-Azhar, a group of Sunni scholars, also said that this is not allowed in Islam and it is important to protect the rights of children.
So, the Al-Azhar Shia fatwa is basically a statement from a group of Muslim scholars in Egypt saying that it is not okay for men to marry young girls who haven't reached puberty yet, even if some shia scholars say it is. They believe in protecting the rights of children and promoting a safe and just society for everyone.