At the top, visitors can step out on to a see-through walkway. 等于：

At the top, visitors can step out onto a see-through walkway.

step out onto 结构：不及物动词+副词+介词，onto 兼有on 和to 两种意思。

又如：

Next week, in Atlantic City, New Jersey, fifty-one beautiful women will step out onto a stage.  下周，在新泽西州大西洋城，51位美貌女郎将登台进行角逐。

The maintenance team has serviced the jet and has it ready to go about an hour before I step out onto the flightline.  在维护团队的努力下飞机已经在我步入到起飞线前的大约一个小时做好了准备。

"This branch looks pretty strong," he said, preparing to step out onto it.   "这根树枝看起来挺结实。"他边说边准备踩上去。

But some believe they gain a psychological advantage over their opponents the moment they step out onto the playing field.  有人认为当他们步入赛场的那一瞬间，就赢得了较对手的优势。

你说的这个句子，在介词from 后面又加了一个介词短语（双重介词），跟刚才讨论的不完全一致。

A man stepped out from behind the wall. 一个人从墙后面走出来。