Hiding your sidebar in Salesforce when you’re not using it is a great way to free up some screen real estate. You can always expand it if you need to see your recent items or the recycle bin.
Here’s how you enable the collapsible sidebar in Salesforce:
Setup | Customize | User Interface
Check the checkbox “Enable Collapsible Sidebar”
Now you can hide or show the sidebar by clicking the hide/show button on your Salesforce pages.