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What does "back up to the top" and "fit past" mean in this sentence?

I'm reading Michael Lewis's The Blind Side. In Chapter Three, Sean Tuohy's daughter Collins talks about Michael Oher.

When she tried to pass him on the stairwell, she said, she had to back up to the top, because she couldn’t fit past him.

What does "back up to the top" and "fit past" mean? I'm not quite sure.

刘竹溪