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 Keyword 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. Select the one(s) you want to apply to your selected contacts and/or type in any new Keywords you want to create and apply in the New Keyword box at the top of this modal. Then click "assign keyword" at the bottom.
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.