Stable coins is a name for the category of coins that have a fixed, non-changeable value. These coins are highly unaffected by market volatility, thus keeping the same price no matter how much selling/buying happens to it
Some of the popular curriencies is the Usdt and the Busd, Each of these coins are pegged to the United states real dollar.
Most of the crypto buyers always buys the USDt with their regular money and then convert the USDT coins to any other Altcoin they prefer like Ethereum, Bitcoin, e.t.c. This helps them prevent losing on the exchange if they bought Altcoins first due to the fluctuation of pricing of these coins.