It finds all of the files with the .rb extension in a directory, then feeds that in to git log --oneline. wc -l then counts how many lines resulted from git log

It finds all of the files with the .rb extension in a directory, then feeds that in to git log --oneline. wc -l then counts how many lines resulted from git log, which basically tells you how many times each file has been modified. Lastly these numbers are sorted from largest number to lowest. The gist of it is that you figure out which files get modified the most. This lets you know where your most frequently changed code is.