Realized loss is the amount of money you lost when you closed a losing trade.

For example, say you took a long position on ETH (you bought ETH) at $200 with $1000 (you have 5 ETH), and the price shot down to $150 and you sold to keep from losing more. Your realized loss would be $250: ($150  - $200) * 5.

Once you've realized a loss, your account balance has decreased, and your drawdown has increased. Try not to worry too much, as trading always involves losing some trades. The key is keeping losses small, and getting much larger wins.