SharePoint 2013 Search :Query Rules Explained

To  promote specific search result ,or for efficient search, relative to user context , query rules are introduced  in SharePoint 2013.
These rules are executed , for particular search terms entered by the site user.
Say  for example, if search result should reflect recently updated site  conversations, you need to define query rule as follows.
1)These rules can be defined at 3 different scopes
For on-premise version, query rules can be defined at farm scope  and for  both the versions(Online version and On Premise version) query rule rules can be defined at tenant (site) and multi-tenant  levels (site collection).

2)Under Site settings-->Search-->Query Rules ,see all the predefined rules by selecting  All Sources (in dropdown) as shown in Image.

3)Identify the predefined query rules ( provided by the SharePoint) which may be close to your requirement.

 and Make the copy of existing one

For this example ,to promote search result for recently added conversations copy predefined query rule for conversion 

4)Edit newly copied query rule  to add the result block for recently updated conversations as follows

  5)Click add result block and Launch query builder to form the search query

6)To know more about the other customization options ,read this and this nice blog post

  and one more nice blog post