When you select Tag contact on an Action tile, you have the option to assign a tag to a contact.
Segments have entry and exit events, i.e. contacts can come in and out of segments depending on their behaviour, whereas tags are simply applied to the contact and used as a filtering tool.
We recommend that you use tagging for permanent statuses, e.g. Staff, VIP, Fraud, etc.
Enter your tag in the With tag field and select Save to finalise.