The HODL amount is the amount of funds that the trade engine will hodl for you. To "hodl" is crypto slang and it means "not selling".
For example, on the market BNB/USDT, the trade engine could hodl BNB for you.
The trade engine will only hodl funds that it acquired itself.
Also, the trade engine will always hodl the most expensively acquired funds. That way it can take profit earlier, on non-hodl funds.
Note that it is possible to have the trade engine hodl funds for you, that it did not acquire itself. To do this, manually add an historic buy order on the orders page for the amount you would like to hodl. This will cause the trade engine to absorb this amount as if it had acquired it itself. After that, it will be able to hodl those funds for you.