Prime numbers: numbers which only have the factors of themselves and 1.
They include numbers such as 17, 19, 23, etc. These are numbers which only have two factors: such as 17, with the factors only being 17 and 1.
So therefore, this means, that for any two prime numbers, unless they are the same number, the only factor they will share is 1. So therefore the GCF of two prime numbers is 1.
Hope this helped!