What are Promises in MongoDB?

I am currently working on a project with mongoDB and am receiving this notification in the terminal:

"DeprecationWarning: Mongoose: mpromise (mongoose's default promise library) is deprecated, plug in your own promise library instead: http://mongoosejs.com/docs/promises.html"

I am fairly new to mongo and have no idea what these "promises" are. I also checked out the link in the notification, but still cannot understand what it is saying.

If someone could please explain what "promises" are within mongodb and what I should do about this deprecation, that would be great. Thanks!

via jblew