A stablecoin is a cryptocurrency pegged to a fiat currency, usually the US dollar; USDT (Tether) is the most used one for paying services because a deposit of 100 USDT is credited as about 100 dollars with no exchange-rate risk.
Paying in Bitcoin means the amount you send is converted at a moving price. Paying in a dollar stablecoin removes that step: the token is worth one dollar by design, so the credited balance matches what you sent, minus nothing but the network fee. USDT exists on several networks; the one you choose decides the fee and the speed.
Stablecoins are the pragmatic choice for a business that budgets in dollars and does not want a treasury position in a volatile coin.
At SMSMeteor: USDT is accepted; the deposit screen shows the network to use, and the USDT page compares the options.