Made of metal, plastic, or rubber, grommets are typically inserted into a fabric hole, such as leather, where they serve to reinforce and protect the hole. Often holes have rough edges, so grommets perform the dual job of protecting the hole from damage and protecting what passes through the hole.
A small grommet may also be called an eyelet, used for example on shoes, tarps and sails for lacing purposes. In electrical applications these are referred to as "insulating bushings". Most common are molded rubber that are inserted into small hole diameters up to 2" in diameter. There are many hole configurations from standard round to assorted U-shapes. Larger penetrations that are irregular in shape as well as long straight edges often use extruded or stamped strips of continuous length. These Continuous length materials are referred to as "grommet edging".
These are quite common in applications that range from telecom switches and data center cabinets to complex and dense wire/cable and even hydraulic tubing in aircraft, transportation vehicles and medical equipment.