The estimate applies self-employment tax (15.3% on net earnings, covering both the employee and employer share of Social Security and Medicare) plus a general federal income tax estimate on top, then divides the annual total into four quarterly payments.
This mirrors how the IRS actually expects freelancers to pay: quarterly estimated payments through the year rather than one lump sum in April, since there's no employer withholding along the way.
The IRS charges an underpayment penalty if you owe more than a small threshold at filing time without having paid quarterly along the way, even if you pay the full balance by the April deadline. Treating quarterly estimates as a real bill, not a suggestion, avoids that penalty and prevents a single overwhelming payment from landing all at once.
Roughly mid-April, mid-June, mid-September, and mid-January of the following year, though exact dates shift slightly year to year: confirm the current IRS schedule before paying.
You may owe an underpayment penalty at filing time, calculated on the gap between what you paid and what you should have paid for that period, even if the full year's tax is eventually paid.
Either is acceptable for the safe-harbor rule (generally 100-110% of last year's tax liability, or 90% of this year's), but if income is growing quickly, basing estimates on this year's actual pace avoids a large balance due at filing.