The line step by step:
position player nearObjects["LaserTarget",1000]
that returns an array of objects of type "LaserTarget" closer than 1000m to the player's position ordered by distance. Of course, if no "LaserTarget" object is found, the returned array will be empty. But if one or more objects of that type were found, they will be placed in the returning array with the closest the first. So:
(position player nearObjects["LaserTarget",1000]) select 0
Selects the first item of the supposedly not empty returned array.
And as it is, in fact, an existing object, with:
position ( (position player nearObjects["LaserTarget",1000]) select 0)
you get the position of the lasertarget object closest to the player.
Now that code has a little problem, because if no lasertarget is present, the returned array of objects would be empty, so you cannot select index 0 from it, much less to get the position of a non-existant array element.
but all in all, with that line of code you get the position of a laser designator target, if any. Now, with that position you may do whatever you want to do. Note also that _postarget is a local variable, so it is suited to be placed inside a script.