The concept of profit is relatively simple: you take the money you've made from selling and deduct all of your expenses and what you're left with is your profit. But in practice it's damned hard to do - just ask the owners of any small business or startup. And the concept itself gets complicated when you consider that some expenses (like rent paid in advance or buying a computer) have an impact in the future. And sometimes you might receive a lump sum as income for services or products that you are going to deliver over a period of time, requiring you to spend money in the future to deliver those items. Various methods of trying to recognize the subtleties of income and expenses have kept accountants in business for centuries.
This stuff can get very complicated but I'm going to try to make it simple. If you're not interested, just remember that you are basically trying to make more money than you are spending - and if you're not you have a problem and you better have plans to fix the problem before you run out of money.