What is a stock?

A stock or share is ownership of a very small part of a publicly traded company. For example one of the most famous and largest publicly trader company is Apple Corporation, ticker symbol AAPL. Apple Corporation has billions of outstanding shares. Most of which are traded on Stock Exchange NASDAQ.  If you have a funded brokerage account, you can pay the price and become a part owner of Apple Corporation.

What is Day Trading?

Day Trading is buying and selling of securities (stock, efts, options, currencies, futures) over a short period of time, typically with in a trading day.
Day Traders try to profit from short term fluctuation in price of stocks or other securities. They tend to buy and sell volatile stocks which are moving on higher than usual volume on some new or other catalyst.