Google have given an option to all users that they can create folders in their Gmail account, it is called labels. Labels are the type of the folders given in your Gmail account.
You can pick your mails according to your preferences. You can create the folders and you can even create sub folders too. This service is applicable on both desktop and mobile phones. Just open your label and see your mails. It’s very easy.
If you delete the labels, your mails will not be deleted. They will remain in the previous sections such as inbox or sent.