This function controls the position along the x-axis of the room of the asset sprite element on the layer. You give the sprite element ID (which you get when you create a sprite element using layer_sprite_create() or when you use the function layer_sprite_get_id()), and then set the x value to use (based on the room coordinates).
Argument Description sprite_element_id The unique ID value of the sprite element to change x The x position for the sprite
var lay_id = layer_get_id("Asset_sky");
var spr_id = layer_sprite_get_id(lay_id, "Clouds");
The above code gets the ID value of the sprite asset "Clouds" assigned to the layer "Asset_sky" and then sets its x position to a random value between 0 and the width of the room.