The Screen Options pull down tab is located in the upper right corner of your Dashboard.
It can be used to show/hide modules on some screens and show/hide columns on other screens. When you click on Screen Options, it will drop down an area with a list of the different things you can show or hide. Uncheck a box to hide something and check it again to show it.
You can also change the number of columns for the modules. Most screens that have configurable modules have an option for one or two columns. The main dashboard page can show up to four columns.
Here is an example of Screen Options to show/hide modules and change the number of columns on the Post Screen:
On screens where you can show/hide columns, you can also change the number of items shown per page. Here is an example from the Media Library screen:
After you are done making changes, click Screen Options again and the pull down area will disappear from the view.
Saving Screen Options
You don’t have to worry about saving any of your Screen Options. WordPress saves your configuration automatically for your in a cookie on your computer and in the database so that when you use another browser or a different computer, everything will look the same.