NADEX is one of many services that deal in "trading" these things called "binary options." Binary options are "contracts" that give the user the option to "buy" or "sell" a settlement contract whether or not the price of something will go up or down by a certain amount at a certain time. If your prediction is correct, you get $100 less any fees (typically the contract price.) If you were NOT correct, you lose the money you bought the "contract" for.
NADEX is a leading American broker in binary options... Or, should I say, scammer. These sort of services are nothing more than high-tech gambling. You aren't really buying or selling anything; You're merely betting on whether or not you can predict the price. You don't own the asset the contract is based off of. These aren't even options at all!
Please, PLEASE stay away from anything related to binary options. They're already wrought with scams, and if somehow they've managed to register with national government's regulatory arms, you're still better off spending your money elsewhere. I strongly suggest going to Robinhood if you're looking to trade assets, or any legitimate exchange such as Poloniex or Bittrex if you're looking to trade crypto. Or just head into your local casino. Nadex and related "brokers" are utter nonsense, and while I haven't lost money with it, I'd rather not see any of my readers fall victim to these things.
Do yourself a favor and stay away.