A Promise is ...

it's a promise to call you back later when the query results are ready.
you ask an asynchronous service to do some work and give it a callback function.
It does that work (in the cloud for example) and eventually it calls your function with the results of the work or an error.

basically, what this means is that, you send a request, and the query gives you a promise to return the results of your query, whether the results is an error or your data

get more info here:
http://exploringjs.com/es6/ch_promises.html