A sutrah is an object used by a person performing Salah 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. 
Sutrah means a screenor cover.
A sutrah is an object used by a person performing Salah as a barrier between himself and one passing in front of him. It is Sunnah for the worshipper, if he is praying alone or is leading others in prayer, to have a sutrah in front of him that will prevent people from walking in front of him, and enable him to focus properly on the actions of the prayer. 
Sahl ibn Abu Hathmah رضي الله عنهnarrated: إذا صلى أحدكم إلى سترة فليدن منها لا يقطع الشيطان عليه صلاته The Prophet Muhammad ﷺsaid: When one of you prays facing a sutrah he should keep close to it, and not let the devil interrupt his prayer. Abu Dawood (number 695); classed as hasan by Ibn ‘Abd al-Barr رحمت اللهin al-Tamheed (4/1505); classed as saheeh by al-Nawawi رحمت اللهin al-Majmoo’ (3/244); classed as saheeh by al-Albaani رحمت اللهin Saheeh Abi Dawood.
And his saying: "Do not perform Salaah except that there is a Sutrah in front of you". Sahih Muslim 499
Narrated Busr bin Saeed رضي الله عنه: It was narrated from Busr bin Sa'eed رضي الله عنهsaid that Zaid bin Khalid رضي الله عنهsent him to Abu Juhaim رضي الله عنهto ask him what he had heard the Messenger of Allah ﷺsay about one who passes in front of a person who is praying? Abu Juhaim رضي الله عنهsaid: "The Messenger of Allah ﷺsaid: "If the one who passes in front of a person who is praying knew what (burden of sin) there is on him, standing for forty would be better for him than passing in front of him." Sunan Nasai Vol. 1:757
Narrated Busr bin Sa`id رضي الله عنه: that Zaid bin Khalid sent him to Abi Juhaim رضي الله عنهto ask him what he had heard from Allah's Messenger ﷺabout a person passing in front of another person who was praying. Abu Juhaim رضي الله عنهreplied, "Allah's Messenger ﷺsaid, 'If the person who passes in front of another person in prayer knew the magnitude of his sin he would prefer to wait for 40 (days, months or years) rather than to pass in front of him." Abu An-Nadr رضي الله عنهsaid, "I do not remember exactly whether he said 40 days, months or years." Sahih al-Bukhari Vol. 1 :489 
Ibn Hajar al-Haytami رحمت اللهsaid:
It may be understood as referring to interruption of the prayer by someone walking in front of him and distracting him, which is mentioned in the hadeeths: interruption of khushoo’ (proper focus) and the harm resulting from being distracted by the one who walks in front, and the shaytaan being able to get control of him by whispering and distracting him from what he is doing so that he does not understand anything of his prayer or anything except a little, so he misses out on the reward. All of that is what is referred to in the hadeeth in which the Prophet ﷺ says: “and not let the shaytaan interrupt his prayer” Al-Fataawa al-Fiqhiyyah al-Kubra (1/170)