Notifications is a way to stay up-to-date with everything that happens with your active publication.
When someone purchased your item, asked a question, etc. you can receive that notification, in order to mantein your backoffice synchronized. For instance to update the stock of your product, answer that question through the API, etc.
To know what events triggers notifications, see MercadoLibre documentation.
GeneXus SDK for MELI includes a procedure called ReceiveNotifications that can be used to receive those notifications messages.
Once deployed, you must set the Notification Callback URL property on your Application Page with that Procedure URL. It will be something like this (for .net):
Then you need to modify the Procedure in order to process the notification messages as you wish.