[plt-scheme] question about continuation example in "t-y-scheme in fixnum days"

Hi,
I'm learning Scheme and got a little bit confused
after reading the tree traversal example on Ch 13,
"Jumps" of Teach yourself scheme in fixnum days. (page
53 in the pdf file).

Could someone enlighten me how (rest-of-tree 'resume),
and also ((null? tree) 'skip) froms work? 
First, what are these functions and how do they work?
Do they take any parameters? How many?

You can see my degree of confusion clearly from my
question ;-). If the question is not clear, I can try
to do better. but I guess you got the idea.

Any help is appreciated.
Thank you in advance and regards.

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