Fair trade is when companies from MEDCs and companies from LEDCs trade products between them with a fair price. It's important that people that work in LEDCs receive the right price, because they need food, they need to pay taxes and so on. Basically, it's necessary so they can live a decent life.
Usually LEDC farmers work very hard to put their products on the market. Almost always, the farmers don't get the amount of money that their products are actually worth. If customers start buying and asking for fair trade products, the farmers will get paid the right value.