The Sutrah (سترة) is a barrier that is placed in front of one who is praying, so that the barrier can prevent anyone from passing in front of the worshipper.
In a Ḥadīth, there states the obligation that there must be a Sutrah within the prayer:
On the authority of Abū Sa'īd (رحمه الله) who said: "The Messenger of Allāh ﷺ said:
'When one of you prays, let him pray toward a sutrah (a barrier). Let him draw near to it and not allow anyone to pass in front of him. And if someone were to come seeking to pass in front of him, then he should fight him. For indeed he is a Shayṭān.'" (1)
In another Ḥadīth related to the Sutrah, by way of ‘Abdullāh ibn as-Samir, reported from Abū Dharr, that he said: "The Messenger of Allāh ﷺ said:
'When one of you stands to pray, then indeed he is to take a sutrah, and should have something in front of him such as the back of his saddle. If he does not have something similar to the back of a saddle to place in front of him, then his prayer can be broken by way of a donkey, a woman or a black dog.'
I said, 'O Abū Dharr, why is it only the black dog? What about the red dog or the yellow dog?' He said, 'My nephew and I asked the Messenger of Allāh ﷺ as you have asked me, and he said:
'The black dog is a Shayṭān.'" (2)
This means that the prayer is invalidated, however it's reward is not diminished. (3)
Sheikh Muqbil ibn Hādī al-Wādiʿī (رحمه الله) was asked: "Is this general to include the passing of a woman in front of a woman?" He answered, "The passing of a woman in front of a woman does not enter into the meaning of the hadīth. For he said within it, 'The prayer of a man is cut..'" (4)
ibn Hazm (رحمه الله) also said:
"Woman do not invalidate the prayer of one another." (5)
So this is in brief the ruling of the Sutrah. May Allāh bless you, and shukran.
1. Sunan ibn Mājah 954
2. Ṣaḥīḥ Muslim 510
3. Sheikh Muqbil رحمه الله was asked regarding the Ḥadīth in Ṣaḥīḥ Muslim 510: "Does this mean that the prayer is invalidated, or its reward is diminished?" Then he responded: "This means that the prayer is invalidated; however, it being rendered invalid does not mean that the reward is diminished." Extracted from The Description of the Prophet's Prayer by Sheikh Muqbil ibn Hādī al-Wādiʿī (page no. 15).
4. Sunan Abū Dāwūd 702, on the authority of Abū Dharr.
5. al-Muhallā 385