what is rake about?

rake about : Examine your installed rails environment.
This command will also detect common installation errors. For example, if it
can’t find a JavaScript runtime, it will provide you with a link to available
runtimes.
Once you get rake about working, you have everything you need to start a stand-
alone web server that can run our newly created Rails application.

What is bundel exec means?

bundle exec is a bundle command to execute a script in the context of the current bundle (the one from your directory's Gemfile)

Your system-wide Gemfile may differ from current Gemfile. You may have newer or older gems with which this script doesn't play nice. This difference in versions can give you weird errors.
Bundle exec helps you avoid these errors. It executes the script using the gems specified in the script's Gemfile rather than the systemwide Gemfile