These labels are a tagging system for emails.
Learn how to use labels to organize messages using Gmail on any operating system.
How to Add a Label to a Gmail Message
Folder-based sorting stores emails in one location.

Label-based sorting can apply several labels to a single message.
Go to the toolbar and select theLabelsicon.
In theLabel Asdialog box, go for the label you want to apply.

ChooseCreate Newto make and apply a new label.
Either click that link, or go toSettings>Labels.
If you’re already logged in to your Gmail account, you could go directly to yourGmail Labels prefs.

At the bottom of the tweaks screen, you’ll see your custom labels.
SelectCreate New Labelto add a new label to the list.
Select theMorebutton to the right of the label name.

Choose the feature you want to change, such as the label color.
Or, selectEditto change the label name or nest it under another label.
SelectSaveto apply the changes.

ChooseRemove Labelfrom theMoremenu to delete a label.
FAQ
you’re able to have up to 500 Gmail labels.
However, you’re free to create additional labels within those labels.

Select all messages in the results and create a new label for the selected messages.