The FDIC is a United States government organisation that protects ordinary money in banks. If your bank belongs to the FDIC and then cannot pay its debts, the FDIC pays you back what you had in that bank, up to a limit. The letters stand for Federal Deposit Insurance Corporation: federal means the US national government; a deposit is money you put in a bank; insurance is a promise to pay when something goes wrong; a corporation here is an official body, not a normal company chasing profit. Banks like to show the FDIC sign in the window so people feel safe leaving money there. Though actually, older learner dictionaries still say the limit is $100,000; since 2008 the usual limit has been $250,000 per person, per bank, per kind of account. It does not protect stocks, or money in a shop that goes bust, or a bank in another country. It is an American safety net, not a world one. You meet it in news about US banks and in business English; people say the letters, not the long name.