Sutrah is something a praying person puts in front of him in order to alert people that they should not pass in front of him while praying. This is according to what the Prophet (peace and blessings be upon him) taught us to do in respect for the person who is praying.
The late Sheikh Ibn `Uthaymeen, a prominent Saudi Muslim scholar, (may Allah shower mercy on his soul) said: If the imam finishes the Prayer and the person who joined the prayer late stands up to complete what he has missed, then in this case he is in fact praying alone, and he has to stop anyone who wants to walk in front of him, because this is what the Prophet (peace and blessings be upon him) enjoined on us. The fact that some people do not stop others from passing in front of them may mean that they are unaware of this ruling, or it may be a matter of misunderstanding, as they think that because they have caught up with the congregation they still come under the ruling of those who are behind the imam even after they have begun to pray alone without the imam. But it is essential for the one who joined the prayer late to stop people from passing in front of him when he stands to make up for what he has missed.