Here's the question you clicked on:
elliet123
what does y=x mean
It means that it's a line, so when y is 1 x is 1 when y is 2 x is 2 and so on
If you look at the plane with a coordinate system, all points in it have x- and y-coordinates. Every point has an x and a y coordinate. For some points, y and x coordinates are equal. If you look for all the points for which this is true, you will see a straight line through the origin, like @walters described above.