You can learn more about Keywords here. In this post I just want to cover how you would go about adding a Keyword to multiple contacts in bulk.
From your Advanced Search page, after running a search that will include your desired contacts, place checks to the left of the names of those contacts to whom you want to add your desired Keyword:
Then, click the Contact Options menu above your search results and select Bulk Actions:
Next, select Assign Keywords from the Bulk Contact Actions modal that appears.
After clicking Assign Keywords, you'll then see the Assign Keywords modal:
The Keywords available here are those that your database users have established within your CRM - you can learn more about adding Keywords to your database here.
To add a Keyword, just begin typing inside the box until you see your desired Keyword, select it and then click Assign Keywords:
In the above example, once you click Assign Keywords, these contacts will have the Keyword "Christmas Card" associated with their contact records.
As you can probably see, it's a quick and easy method to group your contacts in ways that can really help streamline the processes in your office, as you can at any time search for contacts that have the same Keyword assigned to them.