Tags offer a flexible way to organize contacts and can be used in automation triggers and as conditions in automation / workflow. Tags can be added to contacts manually or by automations, forms, link clicks, integrations, and API calls. You can easily use the tags to filter the contacts.
Adding Tags:
You can add a tag to the contact by opening the contact record.
You can add a tag to multiple contacts from the bulk action.
Manage Tags:
You can manage all your tags using the Tag Manager. 
To access the tag manager, please click the profile picture on the top right and select Account Settings--> Tag Manager.
Using the Tag Manager you can edit a tag, delete a tag or add a new tag. You also have the option to select multiple tags and delete them.