What is "sutrah"

A sutrah ( lit., "screen, cover") is an object used by a person performing salat as a barrier between himself and one passing in front of him. The person praying should be positioned at less than or equal to three forearms' length away from the Sutrah. The Sutrah’s height must be no less than one forearm's length (i.e. the height of the rear end of a camel's saddle), whereas its width has no stipulated size. The Sutrah is compulsory for anyone performing salat (unless a Sutrah cannot be found after searching).

While it is apparent that it might appear like the performer is praying to the object in front of him/her, it's not absolutely necessary while praying. It can be substituted by facing a wall or simply blocking the way of a passer with a hand. It is also forbidden for someone to pass in-between a person praying and his Sutrah, or to pass closer than 3 forearms' length to the person who has no Sutrah in front of him (because 3 forearms' length is the furthest distance which one can stay away from a Sutrah). However, a person is allowed to pass between the rows of a congregational salat.