Mouthguards, gum shields, sports guards, they have lots of names but all aim to protect the wearer's teeth, gums and mouth from harm.
There are lots of mouthguards available, the entry level option being the boil and bite ones that can be bought in sports shops or chemists. The boil and bite mouthguards tend to be ill-fitting and inhibit the wearer's ability to talk clearly. They also often encourage a gag reflex due to the guard's ability to move around in the mouth too easily.
It is important for ANY sports person performing a contact sport that requires a mouthguard to get a custom-made mouthguard from a dentist.