Briefly, what causes stock prices to fluctuate day-to-day?

The fluctuation to stock prices is caused by demand and supply. The market forces decided what the price would be. The demand and supply are affected by the the large pool of investors and what they estimate or feel the company's shares worth. 

 Stock prices can rise and fall for a myriad of reasons. When looking at short-term changes in a stock’s price, you need to recognize if the price is the result of a activator or just day to day fluctuations of trading.

if a activator reduces a stock’s price and has no influence on a great company’s earnings or future growth, consider buying at the lower price to get a good deal.

we should monitor on Earnings, Federal Reserve Interest Rates, International Events, market correction