When for combines with a word like each, every, all, some followed by a noun, it acts as an iterator of the members of the specified subset of class [noun].
In respect to each member of the subset of the class...
For every success, the inventor had ten failures.
For each of the first five chapters in the book, write a short summary of the plot.
For every rabbit dispatched, an additional four heads of lettuce went to market.
For every pot there was a lid.
For many a student, this was the first time they had seen snow.