You would like to add a user or lead tag to split people in different audiences.
The most common way our customers use tags is to trigger a funnel message after a conversation for example.
When you finish a conversation with a potential lead, maybe you want to trigger automatically a new funnel to show them some particularity of your product.
If your funnel entry point is based on All Leads with tag = "eBook_funnel" then is so easy for you during a conversation to add this tag to trigger that funnel.
In your conversation sidebar check for the Tags tab and add a new tag by pressing enter as shown below.