Shoes: Stiff soles spread the load of pedaling over a greater surface of your foot, thus increasing both comfort and power transimssion. Finding shoes that fit you well and are comfortable can be like a holy grail... I found them in Nike Cairnes for mtn shoes... but after over 15 years of road riding, I'm still looking for "that perfect" road shoe... There is no perfect shoe for everyone as everyone has different feet.
Shorts: padding in the "saddle area" for comfort and chaffing resistance. There are some other reasons, both those are most important for now. Not sure I should mention the "no underwear" part now or later... :hmmm: Some folks don't like the "exposed" nature of cycling shorts and wear "baggies" which look like cargo shorts but have a cycling short liner sewn in (so the baggy short stays in place when pedaling).
Jersey: tight-fitting, synthetic materials that wick sweat away from your skin, thus keeping you cooler in warm weather and warmer in cold weather (with proper layers).
Socks: Combination of jersey and short attributes, although I've run across some cyclists who ride with none at all.
If you stay with it long enough, it can be amazing how you can find justification for the cost of some of these items...