Mudhar is a financial term that means taking a risk or gambling in an investment. It's like playing a game where you don't know if you'll win or lose.
For example, let's say you have some money and you want to invest in a company. If you invest in a safe company that has been doing well for a long time, that's not mudhar. But if you invest in a risky company that nobody knows much about, that's mudhar. You might make a lot of profit or you might lose all your money.
In Islam, mudhar is not allowed because it's seen as a way of trying to get rich quickly without doing any hard work. Muslims are encouraged to earn money in an honest and fair way, without taking unnecessary risks.