Fenders on bikes are mostly a hard plastic cover on top of a wheel to prevent flaring of any stones, sand, liquid, and other things from getting caught in the front wheel.
It divides the flow of whatever comes and saves the rider from getting sprayed on. It is one of the parts which is usually pre-installed in motorcycles, but if not, then as well it should be considered mandatory by the rider to install these.
The rider can also install these while modifying bike to enhance its looks and functions. The fenders clip onto the bike's seat stays and front fork with bungee-type attachments.
Mountain bikes with a suspension fork can shock board: SKS' fender designed to work specifically with suspension.
Although fenders are fairly common and recommended to be put on all bikes, they do not fall under the one-size-fits-all accessory and can not only differ depending on our riding discipline and bike type but, some bikes may not be compatible with a set of fenders at all.