The locus means the “set of all” so the locus off points where x = y is all points where the x and y coordinate are the same.
One example is (1,1). Another is (2,2). Here the point (x,y) has the same number for x as for y. If you plot a few points (1,1), (2,2), (3,3) you will see they all fall on the same line. It is a diagonal line with positive slope that divides the first and third quadrants exactly in half.
For x=3 we find points where the first number (the x coordinate) is 3. The second number can be anything. Some points are (3,1) (3,2) (3,3). These points all lie on a vertical line that intersects the x axis at 3.
