A stablecoin is a cryptocurrency that's created to maintain a stable value relative to another asset. This asset can be a fiat currency, another cryptocurrency or a commodity. The most common stablecoins are the ones that are pegged to the US dollar. Examples of this are: USDT, USDC and PYPL. An example of a cryptocurrency pegged to gold is PAXG. There are several cryptocurrencies pegged to the value of Bitcoin.
Stablecoins can be a vehicle to invest in an asset without actually purchasing the asset. An example of this would be a gold stablecoin. They can also serve as a means to trade with other countries using stablecoins of the other country's currency.