If a trader supposes that the price of a certain crypto will go up, it's called being bullish. When saying "I'm bullish on BTC," a trader usually means they believe the coin's price will keep increasing. It can also mean that they're buying BTC.
Bullish, bull, and long are often used interchangeably. A crypto trader may say "I am long" or "I am a bull" meaning the same thing. 

Being bearish is the opposite. The "bearish" trader thinks that the price of a crypto-asset will fall. It may mean both the belief and an action. If they act, they may sell the crypto assets they own, or, in other words, "go short."

The origin of these terms is believed to come from the way the animals attack their opponents. A bull thrusts its horns up into the air, while a bear swipes its paws downward. These actions are metaphors for the movement of a market. If the trend is up, it's a bull market. If the trend is down, it's a bear market.

In case you'd like to perform a Bearish or Bullish (or long and short) trade, please make sure that it is to be based on thoroughly tested trading strategies.