In that particular case, the "in" word is a reserved word.
someone, while developing bash, decided to make it so.
the loop structure requires some special words to fit in all the options.
"for" is a reserved words that tells bash, a loop is about to be "declared"
the next param, is the variable that bas will use to hold the step
in tells bash that the next thing is the list to go trough. one loop for each object. on each iteration, the current value is asigned to the variable after the for but before the in
(all this is in the link frankling posted)
that "kid smashing the keybord" is caled "fork bomb"
is a clever way to name functions with weird names (like : ), more data here
Fork bomb - Wikipedia, the free encyclopedia
but the advanced bash scripting guide is very very good, and im sure it has most of what you want.