When you `rails new` in Rails 4, it provides you with these three `bin` files. These are executables which allow you to avoid using `bundle exec` entirely. You would use them like so:

```
# instead of
$ bundle exec rails s
# you would use
$ bin/rails s

# instead of
$ bundle exec rake routes
# you would use
$ bin/rake routes
```

If you have your development built in a certain way, you can add the `./bin` directory to your `PATH` and you could even use `rake routes` and it would automatically use the `bin/rake` file.

Here's a bit more explanation for it: https://github.com/sstephenson/rbenv/wiki/Understanding-binstubs