A stablecoin, like the name suggests, is a type of cryptocurrency that is supposed to have a stable value, such as US$1 per token. How they achieve that varies: the largest, such as tether and USD Coin, are effectively banks. They hold large reserves in cash, liquid assets, and other investments, and simply use those reserves to maintain a stable price.
Others, known as "algorithmic stablecoins", attempt to do the same thing but without any reserves. They have been criticised as effectively being backed by Ponzi schemes, since they require continuous inflows of cash to ensure they don't collapse.
Stablecoins are an important part of the cryptocurrency ecosystem. They provide a safer place for investors to store capital without going through the hassle of cashing out entirely, and allow assets to be denominated in conventional currency, rather than other extremely volatile tokens.
