bitcoin trading is entrust the majority of investment you have to buy be some bitcoin, then you sell it back. such that is rotation of bitcoin. while the risk is you will get a reduced amount of bitcoin as a prediction in the buying and selling bitcoin.
In essence, that's it, but to give a more complete answer to the OP, we have to say why it is so risky. The risk lies in the fact that the BTC market does not have very high liquidity when compared to the most common currencies. This brings some instability to the market, and it is one of the main causes of high volatility. This problem requires time to be resolved, since it is not yet common to see people buying and selling things using Bitcoin.
volatility depends on a lot of factors and not just the liquidity of the market for example the trust that people may have on a particular currency is very important, that is why when you see an economic crisis in a country you will also see a evaluation of their currency.