Belgian nationality law is something that determines if someone is a citizen of Belgium or not. It's like a big rulebook that says who gets to be Belgian and who doesn't.
So, imagine you're in a big club. To be in the club, you have to follow certain rules. In order to be a member of the "Belgium" club, you have to follow certain rules too.
One of the biggest rules is that you have to be born in Belgium or have at least one parent who is a Belgian citizen.
But, there are exceptions to this rule. For example, if you were born outside of Belgium but your parents were both Belgian citizens at the time of your birth, then you can still become a Belgian citizen.
There are also things called naturalization and option. Naturalization is when you apply to become a citizen of Belgium and you fulfill certain requirements, like living in Belgium for a certain amount of time and knowing how to speak one of the national languages. Option is a way for certain people who have a connection to Belgium (like a parent or grandparent) to become a citizen without having to meet all the requirements for naturalization.
It all might seem complicated, but basically, Belgian nationality law decides who gets to be Belgian and who doesn't.