Assalamualaikum ustaz. What is meant by sutrah? And what is the ruling of placing it before one begins his prayer? Hope for an explanation.
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions and all those who follow his footsteps until the Last Day.
Sutrah lexically means anything that obstructs.  In fiqh, the terminology sutrah means anything upright in front of a person praying. It can be a cane, piled soil or others similar in its function to prevent others from passing in front of him. 
In a hadith narrated from Abu Sa’id al-Khudir RA, the Messenger PBUH said:
إِذَا صَلَّى أَحَدُكُمْ فَلْيُصَلِّ إِلَى سُتْرَةٍ ، وَلْيَدْنُ مِنْهَا ، وَلَا يَدَعْ أَحَدًا يَمُرُّ بَيْنَ يَدَيْهِ ، فَإِنْ جَاءَ أَحَدٌ يَمُرُّ ، فَلْيُقَاتِلْهُ ، فَإِنَّهُ شَيْطَانٌ
“The Messenger of Allah (ﷺ) said: ‘When anyone of you performs prayer, let him pray facing towards a Sutrah, and let him get close to it, and not let anyone pass in front of him. If someone comes and wants to pass in front of him, let him fight him, for he is a Shaitan (satan).’” 
The author of the book Fiqh al-Manhaji (1/392-393), stated:
“An obstruction (سُترَةٌ) is a separation sign between a person praying and people walking in front of him, such as a wall, pillar, cane or laid out praying mat and others. If one couldn’t find anything else, then he should just draw a line.”
This is based on the hadith from Ibn Umar RA:
أَنَّ رَسُولَ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ كَانَ إِذَا خَرَجَ يَوْمَ الْعِيدِ أَمَرَ بِالْحَرْبَةِ فَتُوضَعُ بَيْنَ يَدَيْهِ، فَيُصَلِّى إِلَيْهَا وَالنَّاسُ وَرَاءَهُ، وَكَانَ يَفْعَلُ ذَلِكَ فِى السَّفَرِ
“When the Messenger of Allah (ﷺ) would go out (for prayer) on the day of ‘Id, he ordered to bring a lance. it was then set up in front of him and he would pray in its direction, and the people (stood) behind him. He used to do so during the journey; hence the rulers would take it (lance with them).” 
While the prioritized method is by placing a separator near the place of prostration.
This is in accordance with a hadith from Sahal bin Sa’ad RA, he said:
كَانَ بَيْنَ مُصَلَّى رَسُولِ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ وَبَيْنَ الْجِدَارِ مَمَرُّ الشَّاةِ
“Between the place of worship where the Messenger of Allah (ﷺ) prayed and the wall, there was a gap through which a goat could pass.” 
In the book al-Mu’tamad fi al-Fiqh al-Syafie, there is further addition stating the prohibition of a person passing in front of another person praying when he has placed a sutrah in front of him, this follows the hadith of Abu al-Juhaim al-Ansari RA, where the Messenger PBUH said:
لَوْ يَعْلَمُ المَارُّ بَيْنَ يَدَيِ المُصَلِّي مَاذَا عَلَيْهِ، لَكَانَ أَنْ يَقِفَ أَرْبَعِينَ خَيْرًا لَهُ مِنْ أَنْ يَمُرَّ بَيْنَ يَدَيْهِ
“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.” 
Abu al-Nadr, one of the narrators of this hadith said: “I’m unsure whether the Prophet PBUH stated 40 days, 40 months or 40 years.”
It is sunnah for a person praying with a sutrah in front of him to stop anyone from passing in front of him with his hand. This is based on a hadith of Abu Said al-Khudri RA, he heard Rasullullah PBUH said:
إِذَا صَلَّى أَحَدُكُمْ إِلَى شَيْءٍ يَسْتُرُهُ مِنَ النَّاسِ، فَأَرَادَ أَحَدٌ أَنْ يَجْتَازَ بَيْنَ يَدَيْهِ، فَلْيَدْفَعْ فِي نَحْرِهِ فَإِنْ أَبَى فَلْيُقَاتِلْهُ، فَإِنَّمَا هُوَ شَيْطَانٌ
“When one of you prays facing something which conceals him from people and someone wishes to pass in front of him, he should turn away; but if he refuses to go, he should turn him away forcibly, for he is only a devil.” 
As for a person who didn’t place a sutrah in front of him, then he doesn’t have a right to stop anyone from passing in front of him, for he has neglected his own rights by not placing a sutrah. . Likewise, a sutrah for an imam is the sutrah for the makmum.  Hence, in exigent situations, for instance, one’s ablution is invalidated when he is praying and he doesn’t have a choice except to pass in front of the other congregants to retake his ablution, then the other makmum should not stop him.
May Allah grant us knowledge so that we can perform our worships perfectly.