Keywords are descriptive word that are attached to photos in the photo’s metadata. They can be added in Photoshop, Bridge, Lightroom, Darktable and other photo processing programs.
The intent is to make photos easy to find either on the internet or on your hard drive. By adding keywords such as “fruit” or “car” you can search on those terms and find all the files with those keywords.
Keywording is one of those future “I’m glad I did this!” and current “This is a pain.” tasks. The question to consider is how often do you search for photos fitting a certain criteria? If you dabble in selling stock photos, then keywording is essential.
If you want your photos found on the web putting keywords in the metadata is less useful. Search engines don’t focus on the photo’s metadata. You are much better off giving the photos a descriptive name and adding an alt tag.
While descriptive file names are less high tech than metadata, they are often easier to do. They also let you search just by scrolling or using your computer’s search tool.
Not doing either, keywording or creating descriptive file names, may be your default mode and is certainly easiest of all (you don’t have to do anything) but won’t make life easier in the future when trying to find that photo from 5 years ago.