Why do I see Form 1040-ES Payment Vouchers on my federal tax return? I have normal W-2 income via my employer and some capital gains from my brokerage account, that's it.

Why do I see Form 1040-ES Payment Vouchers on my federal tax return? I have normal W-2 income via my employer and some capital gains from my brokerage account, that's it.

I have 4 vouchers, each for an amount of $756. Can't quite figure out why.