What is this guitar accessory on 13th fret?
This is a special capodastro varient with which you can capo individual strings. Such capos are called partial capos. I can't say for sure, but the product name of the one in the video is probably "spider capo".
In the photo it appears that the three higher strings are capo-ed at the 13th or 14th fret while the lower three strings are not affected at this point (but are capo-ed at the 2nd fret). The resulting effect is the same like with a full capo, only with partial capos it is limited to the selected strings.