keeping in mind that a function does not have any X-Repeats, that is, the 1st coordinate in every pair never repeats, that is, the INPUT never repeats, Check the picture below.
now, to check on the graph if it's a function or not, simply recall the vertical line test, if you can draw a vertical line on the graph and it only touches it ONCE, then it's a function, if however a vertical line touches it more than once, then is not.