Taxable accounts are those accounts which are earning are liable to taxes. To be explicit, if you have made any investment in these accounts and you gain profit, a tax would be applicable in this profit. Similarly, the interest earned in these accounts are taxable.
If you have USD 2000 in your account
You invested it and earned USD 200 profit.
If 10% is the Tax rate.
While crediting the account for the USD 200, a tax of 10% would be deducted.
So, USD 20 would settle against tax and only USD 180 would be credited in the account.