Technically, a stock certificate (which was / is a fancy printed piece of paper) indicates ownership of some number (typically 100) of shares of ownership in the company. (money.stackexchange.com)
Since they represent shares of ownership in a company, stocks can appreciate in value if a company performs well, expands, or achieves stronger earnings. (putnam.com)
This can be measured in terms of interest rates and shares of ownership as well as in time, paperwork and plain old hassle. (entrepreneur.com)