1. What is the Sutra? The Sutra is an object (preferably one feet or more in height) used by a person performing Salah as a barrier between himself and one passing in front of him.
2. When we’re praying behind the Imam, during the Fard Salah or Taraveeh Salah. The Imam’s Sutra (barrier) is the barrier for the people praying behind him. It is not allowed to walk in front of the Imam when he’s performing the Salah.
3. However, during the Salah behind the Imam, it is allowed to walk in between the rows, in front of the people who’re praying behind the Imam.
The evidence for that is the report in which ‘Abd-Allaah ibn ‘Abbaas said: “I came along riding on a female donkey one day when I had just reached the age of puberty. The Messenger of Allah ﷺ was leading the people in prayer in Mina, without any kind of wall in front of him. I passed in front of part of the row, then I got down and sent the donkey to graze, and joined the row, and no one rebuked me for that.” (Narrated by Al-Bukhari, 472; Muslim, 504) [See islamqa.info|3404]
4. So if someone walks in between the rows, when we’re praying behind the Imam, it’s not necessary to push them or stop them.
5. When we’re praying alone (i.e. not behind the Imam) it is highly recommended to take a Sutra. No one should be allowed to walk in the area between the Sutra and your feet, it is from the Sunnah to push anyone who tries to cross in front of you in this case.
For more information read: https://www.salafiri.com/sharh-muwatta-imam-malik-shaikh-zubair-ali-zai-hadith-no-48-the-sutra-of-the-imam-is-sufficient-and-equivalent-to-being-the-sutra-for-the-followers/