Depending on what funds / stocks you hold in the taxable account you will received distributions throughout the year which can be classified as dividends, ordinary income, or capital gains depending on the fund.
Capital Gains taxes are assessed when you sell a fund if your basis is less than the selling price. They can also be assessed if the fund distributes capital gains it incurred during the year.
The capital gains rate in the US for capital gains and qualified dividends can be 0%, 15%, or 20% depending on your taxable income.
At the end of the year you will received a 1099 from your broker with the details to record on your tax return.