The 'Keywords' section when uploading your art is for search words and phrases that you don't want to appear in the description but which you would like associated with the art you are uploading. These will be found when someone does a site search but won't be visible to the general public.
For example you may have a landscape but don't want to mention trees, grass and clouds in your main description but would like your art found when someone searches for clouds.
You only have limited characters so use them wisely, try and put yourself in a buyers position and think what they might be looking for that your art offers.
The Keywords work in conjunction with the Categories and Specifications.