Phase 2 Armor is the second full version of armor for Clone Troopers. It was an improvement from Phase 1 armor which was originally deemed too heavy and uncomfortable for most Clones, and it often became damaged easily; aside from helmets not having a reliable ventilation system.
The armor itself proved to eliminate the problems caused with Phase 1 armor, and was proved to be lightweight and compact compared to its predecessor, though it also defended well against blaster bolts.
Phase 2 armor began being developed during the middle of the Clone Wars Era; when the Republic needed more Clone Troopers for combat, as well as a better way of reducing the amount of casualties caused by those which originally wore the somewhat flawed Phase 1 set of armor. It was first worn by the newer generation of Advanced Recon Commandos (ARC Troopers), to whom they wore the experimental version of the armor, as well as demonstrating it on Kamino for Clone Cadet Training.
The armor soon became fully developed by the second-half of the Clone Wars, and proved to have reduced the amount of casualties and outnumber the so-called "defenseless" opposing Battle Droids in the CIS (Confederacy of Independent Systems) or the Seperatists.
Phase 2 armor was eventually replaced from it's lesser-reliable counter-part; Phase 3 armor, to whom the Clone Troopers eventually became known as Imperial Stormtroopers for the 501st Legion. All which occurred after Order 66.
The armor itself first debuted on-screen in Star Wars Episode III: Revenge of the Sith, but first appeared in various sources throughout the Expanded Universe (such as Volume 2 of the Clone Wars microseries).The Experimental Phase 2 ARC Troopers (officers) were the very first Clone Troopers to be issued Phase 2 armor, and served as demonstrators.
Commander Cody was the first Clone Officer introduced to wear Phase 2 armor production-wise, while Commander Colt was the first to wear said armor chronologically (from within the canon).