Trading is one way in which you can make your money work for you. If you understand the concept of buying a house and selling it later when house prices go up, you're already on your way to becoming a trader. The money you make when you sell that house for a higher price is your profit. You don't have to work to earn that profit; you just have to own the house.
The same principle applies to trading. You buy an asset and you sell it at a profit when the time is right. (Actually, the reverse is also possible. You can also make money by selling an asset when its value is declining and buying it back again when it's cheaper – we explain this in our reference on short and long selling in the GT University on this website.)
Buying on credit is a major incentive in modern day trading. Investment banks and brokers will lend you the money you need to make a trade, as long as you can put down a deposit (in trading, that's your margin) and provide proof that you can afford the loan – as you would if, for example, you apply for a bond or enter into a hire purchase agreement. If your trade is successful, you repay the loan and make a profit, and the lender earns a Market Makers Profit Margin on the money you make and earns interest on the loan. So everybody wins.