This is the directory that holds your helpers. In Sails, helpers are shared functions that can be called from anywhere in your app.
When you call sails generate helper tickle-user via the command line from inside your project's root directory, Sails will generate the file api/helpers/tickle-user.js, with a skeleton helper file to get you started.
See the main helpers documentation for more info.
An example of this directory can be found in our sample app repo, here.