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 at top.
Using the "Search Keywords" box at the top you can search for Keywords to add for the contact or create new ones to apply to their record.