Right. The estimated payments are to cover the income tax and self employment tax on your 1099 income since you don't have withholding taken out like from W2 income. It's all combined on your personal tax return you file in April.
For SE self employment tax - if you have a net profit (after expenses) of $400 or more you will pay 15.3% SE Tax on 92.35% of your net profit in addition to your regular income tax on it. So if you have other income like W2 income your extra business income might put you into a higher tax bracket.
