Okay kiddo, so you know how when you have a bunch of toys and you want to organize them, you can put them into groups based on what they have in common? A gcd domain is kind of like that, but for numbers instead of toys.
In math, a domain is a special kind of set of numbers where you can add, subtract, and multiply in a certain way. A gcd domain is a specific type of domain where you can also find something called the greatest common divisor (which we'll call GCD).
The GCD is kind of like the common factor that two numbers share. For example, if you have 6 and 9, their GCD is 3 because that's the biggest number that can go into both of them evenly.
In a gcd domain, you can always find the GCD of any two numbers in that domain. Not every domain has this property, so it makes gcd domains pretty special.
It's important to note that not all numbers are part of a gcd domain. For example, the set of all fractions (like 1/2, 3/4, etc.) is not a gcd domain because you can't always find the GCD of two fractions.
So basically, a gcd domain is a type of set of numbers where you can do some math operations and always find the greatest common divisor of any two numbers in that domain.