When you sell lemonade and make money, you have something called "income." But to make that lemonade, you also had to pay for things like sugar, cups, and maybe even your mom gave you some money to buy those things. The money you spent is called "expenses."
Profit is how much money you have LEFTOVER after you subtract the expenses from the income.
So, let's say you sold $10 worth of lemonade, but you had to buy cups, lemons, and sugar for $5, then your profit would be:
$10 (income) - $5 (expenses) = $5 (profit)
Profit is important because it tells you whether or not your lemonade stand is making money, which is the goal. If you're making more money than you're spending, then you have a profit!