Keywords on an image's metadata are words or tags that describe the image, its contents and subjects, categories or anything else. Keywords can be used when searching for an image or organizing photos. For instance, if I correctly tag all my photos with the name of the people that are in these photos, when I search for "John" I will find all the photos that have "John" in them, so keywords are very useful.
The Image Metadata Manager allows the managing of keywords in a very simple way, presenting existing keywords for single or multiple images and letting you add, edit or remove these keywords. When the keywords section of the metadata is selected, what you can see is something like this:
Image 1 - Keywords
You will notice that the keywords presented in this screen are colourful, most of them are red while one of them is blue. That is because several images are selected and keywords displayed in blue are present in all selected images, while keywords displayed in red are present in one or just some of the selected images. When just one image is selected all the keywords will be presented in black.
To add a keyword, click the 
button and a new keyword will be added to the list, allowing you to edit it immediately. To edit a selected keyword, click the 
button, and to deleted a selected keyword, click the 
button. Please note that any alterations (adding, editing or deleting) in keywords while multiple images are selected will alter the whole list of keywords on all the images, making all of them have the same keywords.