a function means that for every input (value of x), there is only one output (value of y)
y can repeat, x cannot (unless it's with the same y value)
eg:
{(1,2), (1,3), (3,2)} is not a function because 1 has two outputs
but if it was
{(1,2), (2,3), (3,2)} it would be a function. It doesn't matter that 2 repeats in the y coordinate
lmk if you still have questions, I'm happy to help!