SharePoint Online users have a Recycle Bin where deleted content is stored. You can access the Recycle Bin to recover deleted documents and lists if you need to. This is sometimes called the first stage Recycle Bin. If you delete a file from your Recycle Bin the file is taken to a second stage Recycle Bin called the Site Collection Level Recycle Bin.

First stage Recycle Bin

• Accessible from the “Recycle Bin” link on the Quick Launch bar at the top level or sub site level
  
• Available to you if you have Contribute, Design, or Full Control Permissions
• When you delete an item in the list/library, the item is moved to the first level recycle bin. It stays there until it is purged by either you, another user with permissions or automatically deleted after 30 days.
• You can restore the item from the recycle bin or permanently delete the item from the recycle bin
• Items located in this stage counts towards the Site Collection Quota.

Second-Stage/Site Collection Level Recycle Bin

• Accessible from the Site Collection Administration section from the Site Settings page
  
  or via the URL https://[domain].sharepoint.com/_layouts/AdminRecycleBin.aspx
• Available to you if you have the Site Collection Administrators role
• When you delete an item from your first stage recycle bin, it will be moved to the second stage Site Collection recycle bin. It stays there until a site collection administrator deletes it or its automatically deleted after a number of days (up to 30). 
• Example 1:
  If an item was never deleted by the user in the first stage recycle bin, it will be automatically deleted permanently from the first and second stage recycle bin after 30 days
• Example 2:
  If an item is deleted from the first stage recycle bin after 10 days, item will be permanently deleted from the second stage recycle bin after 20 days
• Items deleted from the second recycle bin is not recoverable
• Items located in the second stage doesn’t count towards the Site Collection Quota.
• Site Collection Administrator can manage both recycle bins in a two-views-in-one view
• End-User Recycle Bin Items
• Deleted from End User Recycle Bin
  

Example

You delete a file e.g a document from a document library...

The deleted file goes into your Recycle Bin...

If you delete the file from the Recycle Bin...

 
It goes into the second stage recycle bin – the Site Collection Level Recycle Bin (accessible to you if you have the Site Collection Administrators Role)...

 
When you restore the file from the Site Collection Recycle Bin...

Its restored to its original location...

The following data types are captured by the Recycle Bin:

• Site Collections*
• Sites
• Lists
• Libraries
• Folders
• List items
• Documents
• Web Part pages

The following data types are not captured by the Recycle Bin:

• Site customizations made through SharePoint Designer 2010

 *The Recycle Bin in SharePoint Online for Enterprises provides a safety net when an entire site collection is deleted. When a SharePoint Online administrator deletes a site collection, it is placed in the Recycle Bin where it is kept for 30 days before it is automatically permanently deleted - read more here