When you are uploading email contacts into Privy by importing a .csv file, you can give every contact in that file a tag (or multiple tags) to identify them all in a certain way.
Tagging is different than adding a custom property, because the value will be the same for every contact in that particular csv file that you are importing (as opposed to each contact having possibly different values for the same property). One example is, a custom property would be something like "favorite color" with the values being any color that is available, while a tag would be identifying everyone in an imported file as being retail customers (versus wholesale) or as being from a specific source like instagram.
When you are going through the flow of importing your contacts, you will proceed to a screen that includes an option to add a tag (or multiple tags) to the contacts that you are importing. The tag will be a way to segment or identify the group of contacts that were imported together, so that you can filter for contacts with those tags and email those people at a later time as shown below.
Tags can also be added after the fact by going into a contact profile and adding each desired tag. This requires going into the email contact record and selecting "+ Add a Tag" to edit the tags associated with that contact as shown below.
You have the ability to add multiple tags, update a tag, or delete a tag that no longer pertains to a contact.
Some examples of tags to add are the source of how the contact was collected, like social media, an in-person event, or maybe a form through another app you use other than Privy. You can also tag customers who have purchased from you to identify your customers versus your subscribers who have never purchased from you.