tr is a Unix command that is an abbreviation of translate or transliterate, indicating its operation of replacing or removing specific characters in its input data set.
In Ruby, to get letters in a range say A to M you could pass,
"a".."m", but this will return a range object. However, the String#count method takes in strings. As such
str.count("a".."m") would return an error. However, passing along
"a-m" would not because the latter uses an implementation of the tr Unix command.
TLDR : Ruby strings utilise an implementation of the tr Unix command allowing us to pass strings like
"a-z" and have it unpacked into the entire range without having it as a Range object. Useful in commands that do not take in ranges as arguments.