A stock (also called a share or equity) represents ownership in a company. When you buy a share, you become a part-owner of that business. If it grows, your share grows. If it shrinks, so does your investment.
Stocks have historically produced the highest long-term returns of any mainstream asset class. They also carry the most volatility in the short term.
Mariam buys 10 shares in a halal tech company at £30 each, spending £300. Two years later, the company has grown and each share is worth £42. Her investment is now worth £420. She has earned £120 — a 40% return — by owning a piece of a real business.
Owning shares in companies is generally permissible, provided the company passes Shariah screening. You must avoid companies whose primary business is prohibited (alcohol, gambling, conventional banking, etc.) and those with excessive interest-based debt. The halal ETFs covered in Module 5 automate this screening for you.