When you export, phpmyadmin creates MySQL code which is put into a text (.txt) file. When this code is put into a MySQL command window, or in a PHP script, it will create your tables (table structure) that you exported (and insert data in them if there was any records in the table/s). So when you import, it creates the tables that you exported for backup etc.
Now to import, like Dempey said, click on the SQL image (label 1), a window will pop-up.
You can either copy the text from the exported file and past it into the area (label 2) or click on the "Import" part at the top of the window.
The area in the window will change to that of (label 3). Notice the textbox with "Browse". This, when clicked allows you to find that text file and upload it to the server. The code in the file will then be excecuted and create the tables etc.