One of the sections within the Know Your Client area of contact overviews is the Keywords section, accessible by clicking the "Keywords & Tags" tab at the top of this section:
To add Keywords, click the Add button in the top right corner of the section. You'd then see the below:
Here, you can place a check in the radio box(es) for any available Keywords that you want to add for the contact. Those already attached to their record are checked off and highlighted in blue.