In simple words, volatility means the changes in the price levels of a crypto currency, within a specified time frame ... for example a day or a week.
The higher the volatility, the less secure the currency is considered by some investors as it makes the currency less stable. In other words, traditional investors don't want to buy a bitcoin for $6500 today, just to see it's price to dip below $6000 on the next day.
On the other hand, high volatility also means high possible short term gains, so it attracts the more adventurous investors.
One way or another, many crypto fans agree that the high volatility of crypto currencies in general, make their mass adoption harder.