This is a Newtonian Telescope laser collimator collimation jig.

This jig is used to collimate, or precisely center the laser in the body of the very common model of laser collimator from brands like svbony.

There are tutorials on youtube of the procedure, but here is the gist on how to use it:

You place the laser in the jig and clamp to a table where the laser can fire at a wall 8ft away or so

turn on the laser

tape a piece of paper to the wall and mark where the laser hits

turn the laser in the jig carefully 1/3rd of the way around, mark another pencil mark

repeat until you have a circle of marks. If the marks are far apart ( more than a few mm ) then you can adjust and retry the procedure until the laser beam

does not move more than 1 or 2 mm when you rotate it in the jig.