Unrealized loss is the amount of money you stand to lose if you closed a losing trade at that moment.

For example, say you have a long position on bitcoin (you bought bitcoin) at $10,000 with $1000 (you have 0.1 bitcoin), and the price has gone down to $9,000. Your unrealized loss would be $100: ($9,000  - $10,000) * 0.1 = -$100.

Just remember that a loss means nothing until it has been realized, so try not to get too depressed as long as it's within your stop loss limits.