Question

what does y=x mean

Answer 1

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

Answer 2

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.