Tags are like little markers that you can add to contacts that allow you to later find these contacts more easily.
There is also a special filter you can use to find people that you’ve added tags to.
To add a tag to a contact:
Go to the Relationships tab.
Select the contacts that you want to add a tag to by ticking the box to the left of their names.
Click on More actions > Add a tag.
Type the tag you'd like to add.