A stock trader is someone who buys and sells a piece of a company. It's like buying a bag of candy and then selling some of it to your friends for more money than you paid for it.
In the world of grown-ups, companies can sell pieces of themselves to people like you and me. These pieces are called "stocks". When you buy a stock, you own a little piece of the company. If the company does well, the value of its stocks will go up and you can sell them for more money than you paid for them. If the company does badly, the value of its stocks will go down and you may lose money.
A stock trader is someone who buys and sells stocks for a living. They make money by buying stocks when the price is low and selling them when the price is high. They do a lot of research to figure out which stocks are good to buy and which ones to sell. They watch the news and look at how the company is doing to decide if it's a good time to buy or sell.
Stock traders work on big computers with lots of screens. They look at charts and numbers to keep track of the stock prices. They also work with other traders to try to predict what is going to happen in the stock market. Sometimes they buy stocks for short periods of time - just long enough to make some quick money - and sometimes they buy them for a long time, hoping that the value will go up.
In summary, a stock trader is someone who buys and sells little pieces of a company, hoping to make money by selling them for more than they paid for them. They do lots of research to try to figure out which stocks are good to buy and which are not, and work with others to try to predict what will happen in the market.