Every photo you took with your digital camera contains metadata. In the context of photography, the popular format of metadata is EXIF. Nearly all digital camera manufacturers use EXIF as the format of metadata. EXIF itself contains information regarding a photo like ISO, shutter speed, aperture and so on. The information contained on an EXIF data of a photo can be different depending on the camera settings. Some elements on an EXIF are editable. You can also add additional elements to an EXIF using an appropriate tool.
Typically, RAW editors like Lightroom and Darktable has the capability of editing metadata where you can add additional elements like photo description, author, license, publisher and so on.
In case you are new to Darktable and want to add some additional information to the metadata of your photo, you can do so on the lighttable mode (refer to this post to learn more the basic use of Darktable). Select a photo you want to add the additional information to and click the metadata editor menu.
There are 5 additional information you can add to your photo: title, description, creator, publisher and rights. Once you are done adding the additional information, simply click the apply button to apply the changes.