Just adds a simple border to the outside of an image.
The borders add-on for GIMP automatically adds a clean, one or two-color border to any photo. This is a simple way to make a photo stand out.
To create a single-color border, set the inner and outer border to the same color.
To make sure they are exactly the same, click on one of the colors and copy the text (6 letters/numbers) in the “HTML notation” field. Paste this text into the “HTML notation” field for the other color.