Siahnamayi is a word from the Persian language. In English, it means "black market". Do you know what a market is? It's a place where people can buy and sell things. But the black market is different because it's not legal. That means the things people buy and sell there might be stolen, fake or illegal.
Let's imagine you have a toy store and you sell toys like Lego and Barbie. You buy them from a company that makes them and pays their workers fairly. But sometimes, other people might make fake Legos and Barbies and sell them for cheaper prices. It's not fair to you or the people who made the real toys, is it?
There are also things that are illegal to sell, like drugs or weapons. The black market is where people can buy and sell those things. But it's not safe because those things can cause harm to people. It's important to always buy things legally and safely from trustworthy sources.