Result source is new in SharePoint 2013, using result source we can limit search results based on the search queries.
SharePoint 2010 search scopes are replaced by Result sources in SharePoint 2013.
In SharePoint 2013 provides 16 predefined results are given as shown in the below screen shot.
Local SharePoint Results are set to be a default result source we can also specify existing or custom newly created result source as default for a site collection or site.
To create a result source for a site collection.
Navigate to Site Settings->Search->Result Sources
Click on the Result Source there we can able to create a new result source.
In the General Information section Enter the name.
Click on the Launch Query Builder
By default we can able to see the screen shot shown below.
If you want to search only particular list items in our entire site using this result source.
In the Keyword filter “select only returns items” click on Add Keyword Filter.
In the Property Filter “Select the Path” Contains Manual Value Enter the List URL Click on Add Property Filter.
Click on Test Query we can see the Results Preview,
Keep the default setting as it is.
Click on Save.
Set as default resource source as shown.
Now if we search in our site we can get search results only in that particular list items in our entire site.