What is Function? Function is a rule that acts on one input to make one output.
A non-function happens when there are two or more similar X's. A rule for a function is that there should never be two X's that are the same. Although, there can be as many similar Y's as the writer or mathematician desires.
The example above is not a function because it has two repeating X's.
Functions are made when there are no repeating X's and the one input is affected by a rule and becomes one output.
The example above is a function because it does not have two repeating X's.