map is a function that takes another function as an argument, along with a collection.
map calls the function provided to it on each member of the collection, then returns a new collection with the results of those function calls.
This may feel a strange concept, but it is at the core of Clojure and functional programming.
(map even? [0 1 2 3 4])
Count the number of characters in each word for a collection of strings eg.
["a" "abc" "abcdefg"]
()
(map count ["a" "abc" "abcdefg"])