Stablecoins are cryptocurrencies that have their prices tied to some sort of "stable" asset, such as fiat currency, or exchange-traded commodities (such as precious metals).

They exist for two main reasons:

1. A lot exchanges don't have fiat currencies, they don't have USD, or because of US laws, foreigners can't have USD's. But USD is the world's reserve currency and commodities are priced in USD.
2. Bitcoin and nearly all altcoins are very strongly correlated. If bitcoin goes down, normally all altcoins will go down too. If bitcoin goes up, so do all altcoins. Sometimes the altcoins will break off and act independently, but generally only due to some news event that only affects them temporarily.

Stablecoins allow crypto traders to pull out of volatile cryptocurrencies, like Bitcoin, when prices are falling and take refuge in stablecoins. The most popular stablecoin is called Tether (USDT) which is tied to the US dollar. Tether is very useful as many of the largest exchanges do not have the US dollar as a currency available on their platform, especially for non-Americans.

Note: In many jurisdictions, including the US, trading from cryptocurrency to cryptocurrency (like BTC to USDT) is a taxable event. You can't defer taxes by using Tether.