Short term stock prices are driven by demand and supply. Daily and short term price movement depends on the number of buyers versus the number of sellers.
Many, if not most of them are traders, speculators and very few are real investors.
Their reasons to buy or sell are completely different from those of investors. However, they are in large numbers and trade-in volumes substantially larger than investors.
All of them are strongly influenced by short term events and hence prices fluctuate a lot.
A long term investor need not even try to make sense of these fluctuations. However, they need to know the fair price or value of the stock to take investment decisions.