A bit more complex that you might want as a starting point, but you can see the logic behind the random expression.
Basically, the random expression needs 2 values, a low and a high and it will generate a random value in-between (if u use only value it will assume that you want a number from 0 to that value).
e.g.1 random(20, 30) will produce a number between 20 and 30 (it will be a float value, not an integer)
e.g.2 random(30) will produce a number between 0 and 30.
That way you can create random X and Y coordinates.
e.g.3 every 1 second---> system | create object on layer (whatever) at random(x1, x2), random(y1, y2) whereas x1,x2,y1,y2 can be any number you desire and construct will create the in-between values.
You can also use the layoutHeight and layoutWidth or windowHeight, windowWidth expressions, this way if the layout or the window size change, the coordinates will adapt to the new size relative to the initial dimensions.
e.g.4 random(0, layoutWidth) will produce a random value from 0 to the maximum dimension of your layout, in your case 640 px.