Estimated tax payments are quarterly payments made toward income tax owed for the current year, required when tax isn't otherwise withheld, such as for self-employment income, or for a company's own income tax liability.
The IRS expects tax to be paid roughly as income is earned throughout the year, not all at once at filing time, which is why it charges an underpayment penalty if too little is paid in through the year relative to what's actually owed, even if the full balance is paid by the filing deadline.
For a growing company or a founder with significant self-employment or investment income, estimating these payments accurately means projecting the year's income before it's actually finished, which is inherently imprecise. Most people build in a reasonable safe-harbor cushion, paying at least as much as the prior year's total tax liability, specifically to avoid a penalty even if the current year's estimate turns out to be off.
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