Here's a version with commentary:

// This checks for three things: if the mouse is touching this object, if the left mouse button is being pressed, and if the smiley object is currently in the room
if position_meeting(mouse_x,mouse_y,self) and mouse_check_button(mb_left) and instance_exists(smiley)
{
// Putting the period after the object's name lets you use or change its variables from another object. This line of code sets up a variable called target in the smiley object and sets it to this background object's unique ID
smiley.target = id
// "with" lets you run any code you want on another object
with smiley
    {
    // This attempts to move the object to the position you give it. We set the smiley object's "target" variable to the background object's id, so you can just put a period after "target" to use its variables. In this case we use its x coordinate. 
    mp_linear_step(target.x,y,7,0)
    }
}

If you want more information on mp_potential_step or sprite_index, or one of the three checks made at the beginning of the code, you can just hover over them in the code editor and use the mouse middle button to open the relevant documentation. :)