What is 7005 aluminum bike frame?

Chemically pure aluminum is just about never used in construction, it's too soft. Most of what's labeled as aluminum are really alloys, mixtures between aluminum and small amounts of other metals. 7005 is just the name for a certain "recipe" of aluminum alloy. If you'd look it up you'd see hardness, strength, resistance to fatigue, recommended heat treatments after welding etc etc.