In an IEEE paper template, it has been mentioned that "Keywords should be taken from the taxonomy" and they gave a list of keywords, they have also mentioned elsewhere that these keywords can make an article more easily and reliably discoverable. However, I'm not quite sure whether they are mandatory or not.
If you log into the target journal in IEEE, go to your author center, and click on "start a new submission". On the next page, click the button "Begin Submission". Click on "Step 3: Attributes". This will show you what keywords are required, and how many (look for the "required : X. Max Y") to the right of the Keyword entry. IEEE seems to require between 1 and 3 keywords, and the list could be different per journal. Some also let you enter your own.
You can either keep that draft and use it for your submission, or just delete it when you are done.
If you want to identify the journal, I could give you specifics.