Nasdaq is the second largest stock exchange in the world (by market capitalization of the companies listed under it). Nasdaq and other stock exchange is a place where shares of public listed companies are traded. Through this stock exchange, you could place order on the stocks of companies you would like to purchase/sell.
The NASDAQ is a tech-heavy stock exchange as nearly 50% of the companies in it belong to the tech sector. Companies listed in Nasdaq includes Apple, Microsoft, Netflix, Google, etc.