In the context of the cryptocurrency industry, being bullish is being confident that a particular blockchain platform or cryptocurrency is going to receive a sharp increase in value.
Bullish is the opposite to bearish.
If an investor is feeling bullish about a particular cryptocurrency, they will invest in it at the earliest opportunity. This is because if the value increases as they expect, then the earlier they invest, the more of a return they will make when the value rises.
The reasons for an investor being bullish in the cryptocurrency industry would be almost exactly the same as they would for an individual investing in a company on the stock market. The news cycle is a key indicator, along with the regulatory situations in different countries. For example, prices tend to rise, when countries openly support cryptocurrency.