Imagine you have a bunch of toys, some of them are red and some of them are blue. Your friend also has a bunch of toys, some of them are red and some of them are blue too. Now you both want to keep only the toys that you both have in common, like the toys that are both red and blue.
Similarly, in math, when we talk about the greatest common divisor or GCD, we want to find the biggest number that can divide both of the given numbers without leaving any remainder. This is like finding the toys that you both have in common.
For example, let's say you have 12 toys and your friend has 8 toys. You can count the toys and their colors to find which toys you both have in common: red, blue, and green. So the GCD of 12 and 8 is 4 because 4 is the biggest number that can divide both 12 and 8 without leaving any remainder.
Mathematicians use this concept of GCD in many areas, such as factorizing numbers, simplifying fractions, and finding common denominators. It is like finding common toys that you and your friend can play with together.