The Shulchan Aruch writes (102:4) that one may not walk within four Amot (approximately 6-8 feet) in front of somebody praying the Amidah. The Zohar rules more stringently, and forbids passing within visible distance in front of somebody praying the Amidah, even beyond four Amot. The Ben Ish Chai (Rabbi Yosef Chayim of Baghdad, 1833-1909), in Parashat Yitro, accepts this view of the Zohar, and one should preferably abide by this ruling and refrain from passing within visible distance in front of a person praying the Amidah.
One common example of this Halacha is when a person finishes the Amidah while somebody else still prays the Amidah behind him. One may not step back while reciting "Oseh Shalom" if he will then come within four Amot of the person praying behind him. However, Chacham Ovadia Yosef rules that one may step back to the side of a person reciting the Amidah, so long as he does not come directly in front of him.
Summary: One should avoid passing within visible distance in front of somebody praying the Amidah. Similarly, one should not step back after reciting the Amidah if somebody is praying the Amidah behind him; one may, however, step back to the side of somebody reciting the Amidah.
