Nagel is German for nail -- as in English, this includes the metal peg and the finger/toe nail. However, it is used in English primarily (but still quite rarely!) to refer to a type of hilt guard on a sword or knife.
A hilt may have various structures to protect the hand of a fighter. The most common of these are the paired protrusions of a cross guard or quillon, projecting out below either side of the blade. The nagel is a third protrusion, at a right angle to the quillon and coming out from the center of the back of the hilt just below the base of the blade. This protects the back of the wielder's hand and wrist from cuts, making the flat of the blade more useful for blocking and catching the enemy's blade.
The nagel appears to have originated on small knives that did not have quillons, and functioned to keep one's hand from slipping over the blade while cutting. They first started to appear on quilloned fighting weapons in the in the 14th century and became fairly popular in the 15th century. In the 16th century the nagel was no longer much like a nail at all, becoming a large shell -- often looking very much like a clam shell, and providing more protection to the hand. Alternatively, it might be replaced with a ring, although these rings are not usually called nagels. Eventually these longer nagels and shells evolved into bow hilts and basket hilts. Nagels are most commonly found on the lange messer (in proper German langmesser, lit. long knife), but they might appear on various other types of blades, especially falchions and daggers.
A smaller protrusion -- a lump, rather than a nail-like peg -- would usually be called an écusson or a cusp.