Bearish describes the belief that the price of an asset, or a whole market, is going to fall. A trader who expects declines is "bearish" on that asset and may sell, hold cash or open a short position. The opposite view is bullish. A long period of falling prices is called a bear market, which is explained in full on its own page.
The word is used the same way in stock, currency and cryptocurrency markets. In crypto, bearish signals are often discussed alongside slang such as FUD and "bloodbath".