Stablecoins are a way to use the Ethereum network (or another blockchain) to send value that is pegged to the US dollar. There are a lot of stablecoins now (USDC, DAI, USDT) and they have various mechanisms for staying pegged to the dollar, some more effective than others.

Using stablecoins allows you the advantages of the Ethereum network (interoperability with ethereum loan platforms and other smart contracts) without having to be concerned with the price of ETH.

Stablecoins have the same level of security as their underlying blockchain. To double-spend ethereum-based stablecoins you need to 51% attack Ethereum itself.