The Spam folder is an extremely useful tool for capturing spam and other suspicious email messages and removing them automatically after 30 days. When you first start using Gmail, it's important to check your spam to make sure you see any important messages that have been identified as spam incorrectly.
By default, you need to click on More in the folder list in Gmail to see Spam:
The Spam folder will show - you can drag this folder up above the Less line so that it shows without having to click on More.
You can remove an email from Spam if it has been incorrectly marked it as spam:
In the Spam folder, select the email
Click on Not spam along the top of the screen