Tether is one of the most important trading tools you will find in the crypto market. If you use Poloniex, Kraken or Bittrex then you have seen USDT beforehand. 

Some small details:

• What is USD Tether: USDT is a cryptocurrency asset issued on the BTC blockchain via the OMNI Layer Protocol. Every USD Tether is backed by a USD held by the Tether company which can be redeem via their platform. 
• Current Fiat in the Tether Platform: Yen, USD and Euros
• It's safe: The amount of USDT in circulation can be checked on the Bitcoin blockchain via Omnichest.info. The reserves are published and audited by professionals. Tether Platform also published the bank accounts balance on their website. 

Why is USDT Important:

1. It protects you from volatility: No matter what the cryptocurreny market does, USD Tether will always be at a similar price, as it's anchored to the USD. One USDT is always backed by one USD.
2. It's extremely valuable when the market dips: If the market dips, you want to be ready to buy cheaper crypto currencies. The only way to do so is to have USDT. For more information how to make money with the dips ongoing these days: https://steemit.com/bitcoin/@crypto-insider/bitcoin-will-collapse-this-july-make-money-out-of-it-usdusdusd
3. It minimizes the risk of your portfolio: If you wish to minimize the risk exposure of your portfolio in a specific moment, changing to USDT minimizes highly that risk
4. You can leave your uninvested money without assuming risk: Very often you do not want to take your money out of the cryptospace, and you are also not sure what currency to invest in. USDT suffers no losses, as it's backed, and it's perfect to wait for the correct timing and currency. 
5. Faster than Fiat: If you have USD, EUR or JPY, it will take a while until you can change them for bitcoin. But if you have USDT, EURT or JPYT (Tether currencies), you can immediately change them for bitcoins or other cryptos!