I was reading about relation and function https://www.purplemath.com/modules/fcns.htm
And I was not able to understand this phrase in it where it says "So then this is not a function. Heck, it ain't even a relation!"
I could definitely understand why it isn't a function but couldn't understand why it isn't a relation either. I believe there is no rule for a relation to be a relation. Please explain. Thanks.
It is not a function (as the element 16 of the domain doesn't have an image), but I don't agree with the claim that it is not a relation. A relation between two sets $A$ and $B$ is usually defined just as a subset of $A\times B$.
And the image in your link describes such a subset, namely $$\{(-3,-6),(-2,-1),(-1,0),(0,3),(1,15)\} \subseteq A\times B$$ with $A = \{-3,-2,-1,0,1,16\}$ and $B = \{-6,-1,0,3,15\}$. That is a relation.