Hull plating was an external armor component of a starship's or shuttle's hull which allowed for greater resistance against space debris and enemy weapons fire. They were capable of conducting electricity.
Prior to Earth's development of shields, Starfleet equipped their vessels with the ability to polarize their hull plating, which allowed for increased hull strength in emergency situations. (ENT: "Fortunate Son", "The Catwalk" et al.) However, even into the 24th century some escorts were equipped with ablative hull plating to allow for increased survivability when shields are lost.
The Earth Cargo Service freighter ECS Fortunate set its hull plating to maximum during an attack by Nausicaan pirates in August of 2151. Captain Jonathan Archer also offered first officer Matthew Ryan to upgrade the hull plating and the weapons of the Fortunate. (ENT: "Fortunate Son")
In 2267, Lieutenant Boma expressed his concerns that the Galileo's hull plates wouldn't survive repeated attacks by ape-like creatures. Later, during the electrification of the hull, Commander Spock warned Montgomery Scott not to touch the plates. (TOS: "The Galileo Seven")
The starboard hull plating of the Chaffee was slightly damaged when it entered the energy barrier of a L-class planet in a rescue attempt of Lisa Cusak of the USS Olympia. (DS9: "The Sound of Her Voice")
In 2376, a spatial fluctuation caused one of the hull plates of the Delta Flyer to fly off. Captain Kathryn Janeway asked William Telfer to beam it to the aft section for analysis. (VOY: "Good Shepherd")